feat: add pg_qualstats container image

pg_qualstats is a PostgreSQL extension that collects statistics on query
predicates (WHERE clauses and JOIN conditions). It records execution counts
and selectivity information per operator and column, enabling query tuning
and index recommendation tools like PoWA.

Closes #206

Signed-off-by: Kristiansen, Erling Gustav Moland <Erling.Kristiansen@skatteetaten.no>

Author: Kristiansen, Erling Gustav Moland

pg-qualstats/Dockerfile

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+# SPDX-FileCopyrightText: Copyright © contributors to CloudNativePG, established as CloudNativePG a Series of LF Projects, LLC.
+# SPDX-License-Identifier: Apache-2.0
+
+ARG BASE=ghcr.io/cloudnative-pg/postgresql:18-minimal-trixie
+FROM $BASE AS builder
+
+ARG PG_MAJOR
+ARG EXT_VERSION
+
+USER 0
+
+# Install extension via `apt-get`
+RUN apt-get update && apt-get install -y --no-install-recommends \
+      "postgresql-${PG_MAJOR}-pg-qualstats=${EXT_VERSION}"
+
+FROM scratch
+ARG PG_MAJOR
+
+# Licenses
+COPY --from=builder /usr/share/doc/postgresql-${PG_MAJOR}-pg-qualstats/copyright /licenses/postgresql-${PG_MAJOR}-pg-qualstats/
+
+# Libraries
+COPY --from=builder /usr/lib/postgresql/${PG_MAJOR}/lib/pg_qualstats* /lib/
+COPY --from=builder /usr/lib/postgresql/${PG_MAJOR}/lib/bitcode/ /lib/bitcode/
+
+# Share
+COPY --from=builder /usr/share/postgresql/${PG_MAJOR}/extension/pg_qualstats* /share/extension/
+
+USER 65532:65532

pg-qualstats/README.md

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+# pg_qualstats
+<!--
+SPDX-FileCopyrightText: Copyright © contributors to CloudNativePG, established as CloudNativePG a Series of LF Projects, LLC.
+SPDX-License-Identifier: Apache-2.0
+-->
+
+[pg_qualstats](https://github.com/powa-team/pg_qualstats) is a PostgreSQL
+extension that collects statistics about predicates (quals) found in
+`WHERE` clauses and `JOIN` conditions. It can be used to identify missing
+indexes and understand query workload patterns. For more information, see the
+[official documentation](https://powa.readthedocs.io/en/latest/components/pg_qualstats.html).
+
+## Usage
+
+The `pg_qualstats` extension must be loaded via `shared_preload_libraries`
+to hook into the query executor and collect predicate statistics.
+
+### 1. Add the pg_qualstats extension image to your Cluster
+
+Define the `pg-qualstats` extension under the `postgresql.extensions` section
+of your `Cluster` resource. For example:
+
+```yaml
+apiVersion: postgresql.cnpg.io/v1
+kind: Cluster
+metadata:
+  name: cluster-pg-qualstats
+spec:
+  imageName: ghcr.io/cloudnative-pg/postgresql:18-minimal-trixie
+  instances: 1
+
+  storage:
+    size: 1Gi
+
+  postgresql:
+    shared_preload_libraries:
+    - pg_qualstats
+    extensions:
+    - name: pg-qualstats
+      image:
+        # renovate: suite=trixie-pgdg depName=postgresql-18-pg-qualstats
+        reference: ghcr.io/cloudnative-pg/pg-qualstats:2.1.3-18-trixie
+```
+
+### 2. Enable the extension in a database
+
+You can install `pg_qualstats` in a specific database by creating or updating a
+`Database` resource. For example, to enable it in the `app` database:
+
+```yaml
+apiVersion: postgresql.cnpg.io/v1
+kind: Database
+metadata:
+  name: cluster-pg-qualstats-app
+spec:
+  name: app
+  owner: app
+  cluster:
+    name: cluster-pg-qualstats
+  extensions:
+  - name: pg_qualstats
+    # renovate: suite=trixie-pgdg depName=postgresql-18-pg-qualstats
+    version: '2.1.3'
+```
+
+### 3. Verify installation
+
+Once the database is ready, connect to it with `psql` and run:
+
+```sql
+\dx
+```
+
+You should see `pg_qualstats` listed among the installed extensions.
+
+## Contributors
+
+This extension is maintained by:
+
+- Erling Kristiansen (@egkristi)
+
+The maintainers are responsible for:
+
+- Monitoring upstream releases and security vulnerabilities.
+- Ensuring compatibility with supported PostgreSQL versions.
+- Reviewing and merging contributions specific to this extension's container
+  image and lifecycle.
+
+---
+
+## Licenses and Copyright
+
+This container image contains software that may be licensed under various
+open-source licenses.
+
+All relevant license and copyright information for the `pg_qualstats` extension
+and its dependencies are bundled within the image at:
+
+```text
+/licenses/
+```
+
+By using this image, you agree to comply with the terms of the licenses
+contained therein.

pg-qualstats/metadata.hcl

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+# SPDX-FileCopyrightText: Copyright © contributors to CloudNativePG, established as CloudNativePG a Series of LF Projects, LLC.
+# SPDX-License-Identifier: Apache-2.0
+metadata = {
+  name                     = "pg-qualstats"
+  sql_name                 = "pg_qualstats"
+  image_name               = "pg-qualstats"
+  licenses                 = ["PostgreSQL"]
+  shared_preload_libraries = ["pg_qualstats"]
+  postgresql_parameters    = {}
+  extension_control_path   = []
+  dynamic_library_path     = []
+  ld_library_path          = []
+  bin_path                 = []
+  env                      = {}
+  auto_update_os_libs      = false
+  required_extensions      = []
+  create_extension         = true
+
+  versions = {
+    bookworm = {
+      "18" = {
+        // renovate: suite=bookworm-pgdg depName=postgresql-18-pg-qualstats
+        package = "2.1.3-1.pgdg12+1"
+        // renovate: suite=bookworm-pgdg depName=postgresql-18-pg-qualstats extractVersion=^(?<version>\d+\.\d+\.\d+)
+        sql     = "2.1.3"
+      }
+    }
+    trixie = {
+      "18" = {
+        // renovate: suite=trixie-pgdg depName=postgresql-18-pg-qualstats
+        package = "2.1.3-1.pgdg13+1"
+        // renovate: suite=trixie-pgdg depName=postgresql-18-pg-qualstats extractVersion=^(?<version>\d+\.\d+\.\d+)
+        sql     = "2.1.3"
+      }
+    }
+  }
+}