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.

public/app/plugins/datasource/stackdriver/plugin.json

@@ -19,6 +19,10 @@
       "method": "GET",
       "url": "https://content-monitoring.googleapis.com",
       "jwtTokenAuth": {
+        "scopes": [
+          "https://www.googleapis.com/auth/monitoring.read",
+          "https://www.googleapis.com/auth/cloudplatformprojects.readonly"
+        ],
         "params": {
           "token_uri": "{{.JsonData.tokenUri}}",
           "client_email": "{{.JsonData.clientEmail}}",
@@ -31,6 +35,9 @@
       "method": "GET",
       "url": "https://cloudresourcemanager.googleapis.com",
       "jwtTokenAuth": {
+        "scopes": [
+          "https://www.googleapis.com/auth/cloudplatformprojects.readonly"
+        ],
         "params": {
           "token_uri": "{{.JsonData.tokenUri}}",
           "client_email": "{{.JsonData.clientEmail}}",
@@ -39,4 +46,4 @@
       }
     }
   ]
-}
+}