[runner] Full YAML support

[lucasfcnunes]

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From actions/runner#1764 posted by @sftim on 2022-03-07 (posting here advised by @nikola-jokic). Thanks!

Describe the enhancement

Allow use of YAML as the format for GitHub Actions workflows. Any valid document that can be expressed in YAML 1.x (pick a version you want to support) should be accepted if a conformant YAML implementation converts it to a JSON document, and that JSON document is equal to the transformation result of a “GitHub-flavored YAML” document that GitHub Actions does accept.

Code Snippet

not applicable

Additional information

Prompted by a comment that GitHub Actions doesn't yet support YAML, just a private subset.

NOTE: if the feature request has been agreed upon then the assignee will create an ADR. See docs/adrs/README.md