Add helper to create service accounts used for OIDC authentication

[creydr]

As the Eventing OIDC feature track describes, sending resources use a dedicated service account for their OIDC authentication (which will then be exposed in the AuthStatus).

As different reconcilers will:

• set the .status.auth.serviceAccountName and
• create the service accounts if they don't exist

some helpers would be usefull. This library should include to get the name of the SA (to follow a naming pattern) and to get a SA object (e.g. with annotations, owner references, etc.)