dsproxy: implements support for plugin routes with jwt file

Google Cloud service accounts use a JWT token to get an
oauth access token. This adds support for that.

pkg/plugins/app_plugin.go

@@ -37,8 +37,11 @@ type AppPluginRouteHeader struct {
 	Content string `json:"content"`
 }
 
+// JwtTokenAuth struct is both for normal Token Auth and JWT Token Auth with
+// an uploaded JWT file.
 type JwtTokenAuth struct {
 	Url    string            `json:"url"`
+	Scopes []string          `json:"scopes"`
 	Params map[string]string `json:"params"`
 }