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.

ingress/middleware/verifier.go

@@ -0,0 +1,10 @@
+package middleware
+
+import (
+	"context"
+	"net/http"
+)
+
+type Handler interface {
+	Handle(ctx context.Context, r *http.Request) error
+}