feat: add generic pip build-system support for PEP related to pyproject.toml

Add support for generic pyproject.toml-based projects using pip as build-system,
enabling best-effort installation for backends not explicitly supported.

Features:
- Add 'pip' as new build-system option for generic PEP 517/735 support
- Create virtual environment (.venv) for pip build-system installations
- Support optional pip version upgrade via PYTHON_BUILD_SYSTEM variable for generic pip
- Add PIP_INSTALL_DEPS_OPTS variable for pip-specific install options
- Extend PYTHON_EXTRA_DEPS support to pip build-system using --group flag

Fixes:
- Update Trivy documentation URLs from aquasecurity.github.io to trivy.dev
- Change POETRY_VIRTUALENVS_IN_PROJECT to "true" for consistency with uv, pdm, hatch
- Add .venv to cache paths for performance

Documentation:
- Add comprehensive pip build-system section in README
- Update extra-deps documentation to include pip PEP 735 support
- Clarify best-effort support limitations for release/packaging

Note: Generic pip support works on best-effort basis; release and packaging
jobs may have limitations depending on the specific backend used.