Make KMS logins work the same way in snapshot / online-version-bump

[jku]

Currently GCP KMS and Azure Key Vault are supported differently

• If user wants to use GCP, they
  • define some variables in GitHub settings
  • select GCP when configuring the repo with tuf-on-ci-delegate
• If user wants to use Azure, they
  • define some variables/secrets in GitHub settings
  • select Azure when configuring the repo with tuf-on-ci-delegate
  • AND modify snapshot and version-bump workflows to do azure authentication

I would like it if the workflow modification wasn't necessary. I'm not 100% sure embedding the authentication in the action itself (like we do for GCP) is the correct choice but it would be nice if the solution for both was similar

[jku]

The upside of the GCP solution is that it requires no modification of the online signing workflows which is a plus: upgrading to new workflow versions is easier when they inevitably change.

The downside is that the dependency versions are now managed in this repo: I think this makes sense but I don't know if we can actually test them properly