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/internal/internal.go

@@ -85,3 +85,9 @@ const (
 	// that supports backend returned by grpclb balancer.
 	CredsBundleModeBackendFromBalancer = "backend-from-balancer"
 )
+
+// RLSLoadBalancingPolicyName is the name of the RLS LB policy.
+//
+// It currently has an experimental suffix which would be removed once
+// end-to-end testing of the policy is completed.
+const RLSLoadBalancingPolicyName = "rls_experimental"