Rewrite in pure Python

[b-long]

✨🪩✨ New and Improved! ✨🪩✨

This pull request rewrites this library in "pure python" (of course, using the cryptography library means that Rust is used under the hood for crypto).

In this PR, we introduce a simplified, modernized toolchain. In particular, the CI pipeline and local development is improved using using the uv tool. The most important changes are grouped below.

Build and Test Improvements:

• Switches to uv for Python environment management.

• Switches to building a sub-module using buf and generating Connect RPC client code.

  • See also:
    • Consider generating SDK using buf #45
    • ⚠️ experimentation: buf build #50
    • Consider rewrite in pure Python #54

• Maintains and expands integration testing: separates unit and integration tests (can now be run with uv run pytest, and can execute tests with clear markers).

Cleanup and Redundancy Removal:

• Deletes obsolete workflows related to gopy tooling

These changes significantly simplify the CI/CD process, improve maintainability, and align the workflows with modern Python tooling and best practices.

TODO

• Replace uses of getattr and kwargs
• Remove commented code (see _get_token_from_client_credentials)
• Remove hardcoded realm_name = "opentdf"
• Update README
• Reintroduce cibuildwheel components for multi-arch distribution, and upload built wheels to PyPI

TODO (nice to have)

• Consider adjusting self.issuer_endpoint, to align more closely with Java SDK's SDKBuilder.java (get platform_issuer from well known configuration)
• Add a Changelog
• Add https://github.com/googleapis/release-please-action , including a version number indicating the rewrite milestone
• Enable additional ruff rules ( e.g. "D" for pydocstyle [ docstring conventions] )