Binary Authorization: pass variables needed to create attestations(sign docker images) to gitlab-runner
As part of #7268 we need to have some gcp credentials inside runner in order to create attestations.
See #7268 for details
In this issue we need to:
- Find a way obtain this credentials(we may already have them stored inside cluster information)
- Store them if we don't have them already
- Add predefined variables to runner containing these credentials