Support auto generation of Triggers identity service account and expose in AuthStatus

[creydr]

Problem

As the Eventing OIDC feature track describes, the AuthStatus is meant to provide the generated service account name in the resource status.

When #7173 is done, we should:

• when the feature flag (see Add authentication.oidc feature flag #7174) is disabled:
  • no change in behavior
• when the feature flag (see Add authentication.oidc feature flag #7174) is enabled:
  • create the service account for the Triggers identity
  • expose the service accounts name in .status.auth.serviceAccountName

[aliok]

hi @creydr

I see you've added help-wanted label to this issue. If you think this is a good good-first-issue, can you:

• Comment with /good-first-issue?
• Add clear instructions (including code pointers) about how this issue can be resolved?