All the controllers should use context for signalling termination of communication with API server. Once kcm cancels context all the cert controllers which are started via kcm should cancel the APIServer request in flight instead of hanging around.
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.
