UserInfo contains a uid field alongside groups, username and extra.
This change makes it possible to pass a UID through as an impersonation header like you
can with Impersonate-Group, Impersonate-User and Impersonate-Extra.
This PR contains:
* Changes to impersonation.go to parse the Impersonate-Uid header and authorize uid impersonation
* Unit tests for allowed and disallowed impersonation cases
* An integration test that creates a CertificateSigningRequest using impersonation,
and ensures that the API server populates the correct impersonated spec.uid upon creation.
