TUN-6772: Add a JWT Validator as an ingress verifier

This adds a new verifier interface that can be attached to ingress.Rule.
This would act as a middleware layer that gets executed at the start of
proxy.ProxyHTTP.

A jwt validator implementation for this verifier is also provided. The
validator downloads the public key from the access teams endpoint and
uses it to verify the JWT sent to cloudflared with the audtag (clientID)
information provided in the config.

vendor/google.golang.org/grpc/channelz/channelz.go

@@ -0,0 +1,36 @@
+/*
+ *
+ * Copyright 2020 gRPC authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ *
+ */
+
+// Package channelz exports internals of the channelz implementation as required
+// by other gRPC packages.
+//
+// The implementation of the channelz spec as defined in
+// https://github.com/grpc/proposal/blob/master/A14-channelz.md, is provided by
+// the `internal/channelz` package.
+//
+// Experimental
+//
+// Notice: All APIs in this package are experimental and may be removed in a
+// later release.
+package channelz
+
+import "google.golang.org/grpc/internal/channelz"
+
+// Identifier is an opaque identifier which uniquely identifies an entity in the
+// channelz database.
+type Identifier = channelz.Identifier