ci(actions): add composite actions and clang-tidy PR helper

Add four reusable building blocks that upcoming CI optimization PRs will
consume. No existing workflow is modified; these files are dormant until
referenced.

- .github/actions/setup-ccache: restore ~/.ccache with content-hash keys,
  write ccache.conf with compression and content-based compiler check
- .github/actions/save-ccache: print stats and save the cache under the
  primary key produced by setup-ccache
- .github/actions/build-gazebo-sitl: build px4_sitl_default plus the
  Gazebo Classic plugins with ccache stats between stages
- Tools/ci/run-clang-tidy-pr.py: compute the translation units affected
  by a PR diff and invoke Tools/run-clang-tidy.py on that subset only,
  exiting silently when no C++ files changed

Signed-off-by: Ramon Roche <mrpollo@gmail.com>

.github/actions/save-ccache/action.yml

@@ -0,0 +1,22 @@
+name: Save ccache
+description: Print ccache stats and save to cache
+
+inputs:
+  cache-primary-key:
+    description: Primary cache key from setup-ccache output
+    required: true
+
+runs:
+  using: composite
+  steps:
+    - name: Cache - Stats
+      if: always()
+      shell: bash
+      run: ccache -s
+
+    - name: Cache - Save ccache
+      if: always()
+      uses: actions/cache/save@v4
+      with:
+        path: ~/.ccache
+        key: ${{ inputs.cache-primary-key }}