ci: add rust-style CI workflow

Run `cargo fmt --all --check` on every pull request to enforce
the project's rustfmt.toml. Installs nightly rustfmt explicitly
since rustfmt.toml sets `unstable_features = true`. Per-PR
concurrency group cancels superseded runs.

Signed-off-by: Nikola Pajkovsky <nikolap@openssl.org>

Author: nikolapajkovsky

.github/workflows/rust-style.yml

@@ -0,0 +1,28 @@
+name: Rust Style
+
+on:
+  pull_request:
+
+permissions:
+  contents: read
+
+concurrency:
+  group: rust-style-${{ github.event.pull_request.number || github.ref }}
+  cancel-in-progress: true
+
+jobs:
+  rustfmt:
+    if: github.repository == 'bcgit/bc-rust'
+    name: rustfmt
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@v4
+
+      - name: Install nightly rustfmt
+        run: |
+          rustup toolchain install nightly --profile minimal --component rustfmt
+          rustup override set nightly
+
+      - name: Check formatting
+        run: cargo fmt --all --check