On every PR check if the PR author has signed the CLA against a central list of signers (in `conda/cla` under `cla/signers.json`)

[ForgottenProgramme]

[kenodegard]

See ongoing work:

• Update check-cla action actions#204
  • Add read-file action actions#203

And ongoing testing:

• https://github.com/conda-sandbox/upstream/blob/main/.github/workflows/cla.yml
  • https://github.com/conda-sandbox/upstream/actions/workflows/cla.yml