[BREAKING] Use UserId instead of Username

[agaudreault]

Argo CD 3.2 introduce the Argocd-User-Id header (argoproj/argo-cd#24356) representing the internal subject id used within Argo for authentication. The username is more of a friendly value that is used as a display name in UI and logs.

Ephemeral uses the username as the userId, and to maintain backward compatibility with Argo CD < 3.2, the user Id is optional.

A breaking change should be introduced to make the Username optional and the UserId required. The UserId should be used to retrieve the access requests of a specific subject.

• UserId header should be required [breaking]
• UserId should be a string instead of a string*.
• Username header should be optional, backend should return UserId as Username to client if not provided
• UserId should be used as field indexer
• UserId should be used in AR name generation
• Username should be removed from the AccessRequestKey struct
• Access Binding matching should match the userId instead of the username. [breaking]