ENH: Add platform-specific package install inputs and OS selection

[hjmjohnson]

Summary

Add four optional inputs to the CXX build reusable workflow so that remote
modules with external system-library or platform dependencies can keep using
the standard reusable workflow instead of rolling their own:

• apt-packages — system packages installed via apt-get on Linux runners
• brew-packages — system packages installed via brew on macOS runners
• choco-packages — system packages installed via choco on Windows runners
• os-list — JSON array of runner OS labels to build on (lets modules
  disable platforms that can't work, e.g., macOS for a CUDA module)

Motivation

Modules like ITKIOOpenSlide
require system libraries (e.g., libopenslide-dev) that are not available
on standard GitHub Actions runners. Previously, these modules had to either:

• Disable CI entirely (IOOpenSlide PR COMP: Remove unused action.yml file #37)
• Write a completely custom build workflow

With this change, they can simply pass
apt-packages: 'libopenslide-dev' (plus the matching brew-packages /
choco-packages where applicable) to the reusable workflow.

Usage

jobs:
  cxx-build-workflow:
    uses: InsightSoftwareConsortium/ITKRemoteModuleBuildTestPackageAction/.github/workflows/build-test-cxx.yml@v5.4.6
    with:
      apt-packages: 'libopenslide-dev'
      brew-packages: 'openslide'
      os-list: '["ubuntu-22.04", "macos-15-intel", "macos-15"]'

Multiple packages can be space-separated:
apt-packages: 'libopenslide-dev libfftw3-dev'.

Details

• Packages are installed via sudo apt-get install -y -qq (Linux),
  brew install (macOS), or choco install -y (Windows)
• The install step is skipped on runners that don't match the input's
  platform (e.g., apt-packages is a no-op on macOS/Windows)
• The step is skipped entirely when the corresponding input is not
  provided — no impact on existing modules
• README's short "Example Usage" block now documents the explicit
  secrets: pypi_password: ${{ secrets.pypi_password }} whitelist
  form, matching the detailed breakdown later in the same README and
  following the principle of least privilege rather than
  secrets: inherit

Test plan

• Update ITKIOOpenSlide PR COMP: Remove unused action.yml file #37 to use apt-packages: 'libopenslide-dev' and re-enable the CXX build

🤖 Generated with Claude Code

[dzenanz]

Looks good.

[thewtex]

@hjmjohnson thanks!!

A change requested inline.