Handle 'git push -o ci.skip'

Now git push -o ci.skip, just like including [ci-skip] in the commit message of the HEAD commit, the pipeline will be created, but marked as skipped:

Also, only ci.skip is supported; no other variants are accepted. Since this is a new feature, and AFAIK no other CI services support this feature, I see no reason to accept anything else.

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What does this MR do?

This MR implements git push -o ci.skip, which will skip a CI pipeline via push option, rather than [ci-skip] in the commit message.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• Internationalization required/considered

What are the relevant issue numbers?

Requires gitlab-shell!166 (merged) to be functional, although an using older gitlab-shell will have no negative impact.

Closes #18667 (closed)