CI Lint API support setting branch or SHA when using Validate a project's CI config endpoint

Proposal

Validate a project's CI config API endpoint currently only allows checking the config from HEAD of the default branch. It would be very nice to be able to set the branch (or even a specific SHA) from which the configuration should be validated. It would:

• simplify checking CI configuration in CI pipelines
  • No need for jq and curl or similar for passing the config file to API
• also solve the problem of not being able to pass multiple files for linting (see #349696)