Add trusted builder key to Endorsement Release Authorization Logic Policy

[aferr]

From what @rbehjati tells me, the outputs of trusted builders like Github Actions and Google Cloud Build will be signed using a unique keypair for these trusted builders. This signing does not yet exist (even independently of authorization logic), but we should add the relevant keys into the authorization logic policy once the signing is set up.

I think I heard that @mariaschett is working on adding key management for this (?).

Do either of you have more relevant details for this?

[rbehjati]

We have this issue on Oak for tracking the signing of the Oak provenances: project-oak/oak#2626, but we are still working out the details.