Add support for RSA private key for signing CI JWTs

What does this MR do?

Add support for providing RSA private key that will be used to sign CI_JOB_JWT (if not provided it will be auto-generated).

Related issues

gitlab#214607 (closed)

Checklist

See Definition of done.

• Changelog entry created. Not applicable for Documentation changes and minor changes.
• Documentation created/updated
• Tests added
• Integration tests added to GitLab QA, if applicable
• MR targeting master branch
• MR has a green pipeline on GitLab.com
• Equivalent MR/issue for CNG opened if applicable
• trigger-package has a green pipeline running against latest commit

Reviewer Checklist

In addition to above, reviewer must:

• Pipeline is green on dev.gitlab.org if the change is not touching documentation or internal cookbooks