feat: extract flagd evaluator into api, core, and testkit packages

Split the flagd evaluation logic from the provider into three
independent packages under tools/, mirroring the Java SDK contrib
architecture (PRs #1696 and #1742):

- openfeature-flagd-api: Evaluator Protocol defining the contract
  for flag evaluation implementations
- openfeature-flagd-core: Reference implementation with FlagdCore
  class, targeting engine, and custom operators (fractional, sem_ver,
  starts_with, ends_with)
- openfeature-flagd-api-testkit: Compliance test suite bundling
  gherkin feature files from the test-harness evaluator directory

The provider's InProcessResolver now delegates to FlagdCore via an
adapter pattern, keeping connector code (FileWatcher, GrpcWatcher)
unchanged. Old provider modules (flags.py, targeting.py, custom_ops.py)
are thin re-exports from the core package for backward compatibility.

Also updates the test-harness submodule from v2.11.1 to v3.5.0.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Signed-off-by: Simon Schrottner <simon.schrottner@dynatrace.com>

Author: aepfli

.gitmodules

@@ -6,4 +6,5 @@
 	url = https://github.com/open-feature/spec
 [submodule "providers/openfeature-provider-flagd/openfeature/test-harness"]
 	path = providers/openfeature-provider-flagd/openfeature/test-harness
-    url = https://github.com/open-feature/flagd-testbed.git
+	url = https://github.com/open-feature/flagd-testbed.git
+	branch = v3.5.0

.release-please-manifest.json

@@ -5,5 +5,8 @@
   "providers/openfeature-provider-ofrep": "0.2.0",
   "providers/openfeature-provider-flipt": "0.1.3",
   "providers/openfeature-provider-env-var": "0.1.1",
-  "providers/openfeature-provider-unleash": "0.1.2"
+  "providers/openfeature-provider-unleash": "0.1.2",
+  "tools/openfeature-flagd-api": "0.1.0",
+  "tools/openfeature-flagd-core": "0.1.0",
+  "tools/openfeature-flagd-api-testkit": "0.1.0"
 }