Allow seed job scm to be configured with github-branch-source/GitHubAppCredentials

[andlaz]

Describe the solution you'd like
I'd like for the operator to allow GitHubAppCredentials type credentials to configure the git SCM for seed jobs. This is already supported in the git SCM, i suppose because GitHubAppCredentials implements StandardUsernamePasswordCredentials ..but i'm not sure. I know i can update the credentials on the operator-created seed job ( with no other changes ) and scan and checkout will both work.

Describe alternatives you've considered
Moving away from personal access tokens to github apps, there isn't an alternative it seems: I currently configure the seed job with an app client secret ( username + password ) that validates in the operator but fails to authenticate at scan-time. I log in to the UI, update the seed job credential to the GitHubAppCredentials one ( appid + private key ) and everything works.

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