feat(release): Automated trainer release process

[milinddethe15]

What this PR does / why we need it:
To release a newer version of trainer, user has to run make release VERSION=1.0.0 GITHUB_TOKEN=<token> and open PR with the generated commit.

• Release PR check: validate semver, ensure tag doesn’t exist and verify manifests, chart version, and Python API version match VERSION.
• Release workflow: create release branch/tag, build Python API dist, publish it to PyPI (requires PYPI_API_TOKEN secret in repo) and create a GitHub release using git-cliff-generated changelog.

Which issue(s) this PR fixes (optional, in Fixes #<issue number>, #<issue number>, ... format, will close the issue(s) when PR gets merged):
Fixes #2155

Checklist:

• Docs included if any changes are user facing

[google-oss-prow]

[APPROVALNOTIFIER] This PR is NOT APPROVED

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Details

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[coveralls]

Pull Request Test Coverage Report for Build 21821785755

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

• For more information on this, see Tracking coverage changes with pull request builds.
• To avoid this issue with future PRs, see these Recommended CI Configurations.
• For a quick fix, rebase this PR at GitHub. Your next report should be accurate.

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 2 unchanged lines in 1 file lost coverage.
• Overall coverage increased (+0.8%) to 51.998%

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Files with Coverage Reduction	New Missed Lines	%
pkg/runtime/framework/plugins/registry.go	2	0.0%

Totals
Change from base Build 21715897523:	0.8%
Covered Lines:	1288
Relevant Lines:	2477

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💛 - Coveralls

[jaiakash]

/retest