feat(option): officially deprecate ImpersonateCredentials (#1683)
feat(option): officially deprecate ImpersonateCredentials (#1683)

This option has been soft deprecated for a long time now. Lets make
it official to help more users migrate.
googleapis/google-api-go-client,google/google-api-go-client,googleapis/google-api-go-client,google/google-api-go-client