auth: Refactor how authentication methods are passed to server/auth library.

server/auth/middleware.go

+// Copyright 2017 The LUCI Authors. All rights reserved.
+// Use of this source code is governed under the Apache License, Version 2.0
+// that can be found in the LICENSE file.
+
+package auth
+
+import (
+        "github.com/luci/luci-go/server/router"
+)
+
+// Authenticate returns a middleware that performs authentication.
+//
+// This is simplest form of this middleware that uses only one authentication
+// method. It is sufficient in most cases.
+//
+// This middleware either updates the context by injecting the authentication
+// state into it (enabling functions like CurrentIdentity and IsMember), or
+// aborts the request with an HTTP 401 or HTTP 500 error.
+//
+// Note that it passes through anonymous requests. CurrentIdentity returns
+// identity.AnonymousIdentity in this case. Use separate authorization layer to
+// further restrict the access, if necessary.
+func Authenticate(m Method) router.Middleware {
+        a := &Authenticator{Methods: []Method{m}}
+        return a.GetMiddleware()
+}