Add ability to pull batch URLs from API discovery document

[erickoledadevrel]

Newer APIs don't use the default batch URL ("https://www.googleapis.com/batch"). For example, the Classroom API uses "https://classroom.googleapis.com/batch", as indicated in it's discovery document:

{
  ...
 "baseUrl": "https://classroom.googleapis.com/",
 "basePath": "/",
 "rootUrl": "https://classroom.googleapis.com/",
 "servicePath": "",
 "batchPath": "batch",
  ...
}

Currently, developers must know to specify this URL when creating a batch request:

var batch = new BatchRequest(service, "https://classroom.googleapis.com/batch");

Ideally the client library would automatically determine the correct batch URL based on the service provided.