[Feature] Add automated PyPI publishing workflow

[ylangtsou]

Description

This commit introduces a complete GitHub Actions workflow for automatically building and publishing the Python package to PyPI.

• Adds the release.yml workflow, supporting multiple triggers: Tag push and Schedule.
• Adds the determine_release_vars.sh script, which contains the core versioning logic.

Tests

github

Checklist

Before submitting this PR, please make sure:

• I have performed a self-review of my code.
• I have necessary comments in my code, particularly in hard-to-understand areas.

[github-actions]

Description

Start with a short description of what the PR does and how this is a change from
the past.

The rest of the description includes relevant details and context, examples:

• why is this change being made,
• the problem being solved and any relevant context,
• why this is a good solution,
• some information about the specific implementation,
• shortcomings of the solution and possible future improvements.

If the change fixes a bug or a Github issue, please include a link, e.g.,:
FIXES: b/123456
FIXES: #123456

Tests

Please describe how you tested this change, and include any instructions and/or
commands to reproduce.

Checklist

Before submitting this PR, please make sure:

• I have performed a self-review of my code.
• I have necessary comments in my code, particularly in hard-to-understand areas.
• I have made or will make corresponding changes to any relevant documentation.

[jcyang43]

Your current setup will lead to triggering the release workflow even if the tag is created on a non-main branch.

We only want the release tag to go off if it's created on the protected (main) branch. This is required to ensure only admins or selected people can trigger a stable release via release tag