Provide OIDC discovery endpoints

- Add handlers for service account issuer metadata.
- Add option to manually override JWKS URI.
- Add unit and integration tests.
- Add a separate ServiceAccountIssuerDiscovery feature gate.

Additional notes:
- If not explicitly overridden, the JWKS URI will be based on
  the API server's external address and port.

- The metadata server is configured with the validating key set rather
than the signing key set. This allows for key rotation because tokens
can still be validated by the keys exposed in the JWKs URL, even if the
signing key has been rotated (note this may still be a short window if
tokens have short lifetimes).

- The trust model of OIDC discovery requires that the relying party
fetch the issuer metadata via HTTPS; the trust of the issuer metadata
comes from the server presenting a TLS certificate with a trust chain
back to the from the relying party's root(s) of trust. For tests, we use
a local issuer (https://kubernetes.default.svc) for the certificate
so that workloads within the cluster can authenticate it when fetching
OIDC metadata. An API server cannot validly claim https://kubernetes.io,
but within the cluster, it is the authority for kubernetes.default.svc,
according to the in-cluster config.

Co-authored-by: Michael Taufen <mtaufen@google.com>

Kubernetes-commit: 5a176ac77241ff059f22609fc569ac219334238c

pkg/server/options/server_run_options.go

@@ -180,7 +180,7 @@ func (s *ServerRunOptions) AddUniversalFlags(fs *pflag.FlagSet) {
 		"Memory limit for apiserver in MB (used to configure sizes of caches, etc.)")
 
 	fs.StringVar(&s.ExternalHost, "external-hostname", s.ExternalHost,
-		"The hostname to use when generating externalized URLs for this master (e.g. Swagger API Docs).")
+		"The hostname to use when generating externalized URLs for this master (e.g. Swagger API Docs or OpenID Discovery).")
 
 	deprecatedMasterServiceNamespace := metav1.NamespaceDefault
 	fs.StringVar(&deprecatedMasterServiceNamespace, "master-service-namespace", deprecatedMasterServiceNamespace, ""+