ci: add manually triggerable beta release workflow

[vdusek]

Summary

• Extract the beta release pipeline (release_prepare → changelog_update → pypi_publish → doc_release_post_publish) from on_master.yaml into a new manual_release_beta.yaml reusable workflow.
• The new workflow exposes both workflow_dispatch (manual trigger from the GitHub UI) and workflow_call (so on_master can keep auto-publishing betas after master pushes).
• on_master.yaml now has a single beta_release job that calls the new workflow, gated on the same feat|fix|perf|refactor|style: commit prefixes and on code_checks + docstrings_checks + tests passing.

Why

We previously had no way to cut a beta without pushing a release-triggering commit to master. With this change, a beta can be released on demand from the Actions tab, while the existing automatic flow on master is preserved.

Mirrors apify/apify-sdk-python#886.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.50%. Comparing base (64998e8) to head (ec44d82).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #775      +/-   ##
==========================================
- Coverage   97.54%   97.50%   -0.05%     
==========================================
  Files          47       47              
  Lines        4725     4725              
==========================================
- Hits         4609     4607       -2     
- Misses        116      118       +2     

Flag	Coverage Δ
integration	95.40% <ø> (+0.12%)	⬆️
unit	82.11% <ø> (ø)

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