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 + 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 92.45%. Comparing base (6b78f38) to head (9ec381e).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1868      +/-   ##
==========================================
+ Coverage   92.42%   92.45%   +0.02%     
==========================================
  Files         158      158              
  Lines       11027    11027              
==========================================
+ Hits        10192    10195       +3     
+ Misses        835      832       -3     

Flag	Coverage Δ
unit	92.45% <ø> (+0.02%)	⬆️

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