Update pg16-merge-validation workflow to use the correct
cloudberry-env.sh script name for sourcing environment variables.

Replace bundled Python source packages with pip3 download to comply with
Apache Release policy.

- Remove 7 bundled tar.gz packages from gpMgmt/bin/pythonSrc/ext/
- Add download-python-deps target using pip3 download
- Update build targets to depend on download step
- Preserve original compilation logic and --with-pythonsrc-ext functionality
- Update LICENSE, python-dependencies.txt, and Windows build script
- Add .gitignore rules for downloaded files

This maintains the same build behavior while ensuring Apache compliance
by removing third-party packages from source tree.

See: apache#961

Replace pip3 download commands with curl for downloading psutil,
PyYAML, PyGreSQL and mock packages in gpMgmt/bin/Makefile.

This change unifies the download mechanism to use curl consistently
for all Python dependencies, avoiding potential pip3 download
compatibility issues across different environments.

Changes:
- Remove pip3 download commands for psutil, PyYAML, PyGreSQL & mock
- Use curl to download packages directly from PyPI
- Maintain existing wheel and cython installation for PyYAML build
- Keep the same package versions and download locations

Also, add curl and pip3 dependency detection in configure.ac file for
better user experience. When `--with-pythonsrc-ext` option is enabled,
configure can check the required dependencies early.