Add support App Engine Admin API

[JustinBeckwith]

We have an Admin API (service management) for App Engine here:
https://cloud.google.com/appengine/docs/admin-api/reference/rest/

It's a pretty basic CRUD / REST style API, but it would be great to have wrappers that make it easier to use.

[stephenplusplus]

Added it to the list here: #1338.