ENH: Add platform-specific package install inputs for system dependencies

Add optional inputs to the CXX build workflow for installing system
packages on each runner platform before building:

- apt-packages: Linux (apt-get install)
- brew-packages: macOS (brew install)
- choco-packages: Windows (choco install)

This enables modules with external library dependencies (e.g.,
ITKIOOpenSlide requiring libopenslide-dev / openslide) to use
the standard reusable workflow instead of a custom build workflow.

Usage:
```yaml
cxx-build-workflow:
  uses: .../build-test-cxx.yml@v5.4.6
  with:
    apt-packages: 'libopenslide-dev'
    brew-packages: 'openslide'
```

Each step only runs on its respective platform and is skipped
entirely when the input is not provided.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: hjmjohnson

.github/workflows/build-test-cxx.yml

@@ -30,6 +30,18 @@ on:
         # example: MeshToPolyData@3ad8f08:BSplineGradient@0.3.0
         required: false
         type: string
+      apt-packages:
+        description: 'Space-separated list of apt packages to install on Linux runners before building (e.g. libopenslide-dev libfftw3-dev)'
+        required: false
+        type: string
+      brew-packages:
+        description: 'Space-separated list of Homebrew packages to install on macOS runners before building (e.g. openslide libomp)'
+        required: false
+        type: string
+      choco-packages:
+        description: 'Space-separated list of Chocolatey packages to install on Windows runners before building'
+        required: false
+        type: string
 
 jobs:
   build-test-cxx:
@@ -65,6 +77,22 @@ jobs:
       with:
         large-packages: false
 
+    - name: Install system packages (Linux)
+      if: inputs.apt-packages != '' && runner.os == 'Linux'
+      run: |
+        sudo apt-get update -qq
+        sudo apt-get install -y -qq ${{ inputs.apt-packages }}
+
+    - name: Install system packages (macOS)
+      if: inputs.brew-packages != '' && runner.os == 'macOS'
+      run: |
+        brew install ${{ inputs.brew-packages }}
+
+    - name: Install system packages (Windows)
+      if: inputs.choco-packages != '' && runner.os == 'Windows'
+      run: |
+        choco install -y ${{ inputs.choco-packages }}
+
     - name: Set up Python 3.11
       uses: actions/setup-python@v5
       with: