endpoints: add /auth/info/firebase endpoint

[broady]

This one needs CORS headers.

See sample implementation here:
https://github.com/GoogleCloudPlatform/golang-samples/blob/master/docs/managed_vms/endpoints/app.go

and copy the new swagger file:
https://github.com/GoogleCloudPlatform/golang-samples/blob/master/docs/managed_vms/endpoints/swagger.yaml

[lesv]

Obsolete