ci(workflow): add Puppetcore gem sources validation

- Add reusable workflow for validating gem source configuration
- Configure proper authentication for private gem source
- Set up Ruby environment with caching optimizations
- Run gemfile_spec verification tests
- Support workflow dispatch with branch selection

This workflow follows the central repository pattern for CI resources
and implements the documented authentication pattern for Puppetcore gems.

Signed-off-by: Gavin Didrichsen <gavin.didrichsen@gmail.com>