ci: add release-plz to handle release pipeline

[kalvinnchau]

From: https://release-plz.dev/docs/github/quickstart, just includes the published release part and not the unpublished packages section.

This adds https://github.com/release-plz/release-plz to handle releases for the rust-sdk.
It'll create a PR that will stay updated with commits that are merged, then once we're ready to release we merge that PR.

Motivation and Context

Automate the release process.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update

Checklist

• I have read the MCP Documentation
• My code follows the repository's style guidelines
• New and existing tests pass locally
• I have added appropriate error handling
• I have added or updated documentation as needed

Additional context