ci: implement complete GitHub Actions pipeline (Build, CodeQL, Cppcheck)

[rkt0209]

Description

This PR modernizes the libcupsfilters repository by introducing a complete, multi-layered GitHub Actions CI pipeline.

CI/CD Workflows Added

• Build and Test (build.yml): Automatically installs dependencies, runs Autotools, and ensures the codebase builds cleanly.
• CodeQL Analysis (codeql.yml): Implements GitHub's advanced static analysis scanner to catch complex security vulnerabilities in the C/C++ code.
• Cppcheck (cppcheck.yml): A fast, pre-compilation static analysis step configured to catch critical memory and null-pointer bugs.

⚠️ Note on Cppcheck Status

The cppcheck workflow is currently failing because it successfully caught pre-existing bugs in c code.

I am leaving this workflow as-is to demonstrate that the CI is working correctly. I will open a separate, follow-up PR immediately after this one to patch the C bugs and turn this workflow green.

[rkt0209]

Just a quick heads-up regarding the failing cppcheck workflow in this PR: the CI configuration is working exactly as intended, but it successfully caught several pre-existing static analysis warnings and memory leaks across the codebase.

To keep this PR focused purely on the CI setup, I have opened a separate PR specifically dedicated to fixing those C code bugs here: #133

Once that bug-fix PR is reviewed and merged, the cppcheck workflow on this branch will pass cleanly, and we can safely merge this CI configuration!

[github-advanced-security]

This pull request sets up GitHub code scanning for this repository. Once the scans have completed and the checks have passed, the analysis results for this pull request branch will appear on this overview. Once you merge this pull request, the 'Security' tab will show more code scanning analysis results (for example, for the default branch). Depending on your configuration and choice of analysis tool, future pull requests will be annotated with code scanning analysis results. For more information about GitHub code scanning, check out the documentation.