Add messaging send_all and send_multicast functions - #283

firebase_admin.messaging
  send_all(messages, dry_run=False, app=None): BatchResponse
  send_multicast(multicast_message, dry_run=False, app=None): BatchResponse

  class MulticastMessage
    tokens: list<str>
    Everything else on Message class except token, topic and condition

  class BatchResponse
    responses: list<SendResponse>
    success_count: int
    failure_count: int

  class SendResponse
    success: boolean
    message_id: str
    exception: ApiCallError

import json

import googleapiclient
from googleapiclient import http
from googleapiclient import _auth
import google.auth

# Init Google Credentials (this part is currently handled by firebase_admin.credentials)
_scopes = [
    'https://www.googleapis.com/auth/cloud-platform',
    'https://www.googleapis.com/auth/datastore',
    'https://www.googleapis.com/auth/devstorage.read_write',
    'https://www.googleapis.com/auth/firebase',
    'https://www.googleapis.com/auth/identitytoolkit',
    'https://www.googleapis.com/auth/userinfo.email'
]
creds, _ = google.auth.default(scopes=_scopes)

# It sucks that we have to use this internal _auth module. Wish there was a better way
transport = _auth.authorized_http(creds)

responses = []

def callback(req_id, resp, exception):
  print(req_id, resp, exception)
  responses.append(resp['name'])

def postproc(resp, body):
  """This gets called for each part. It's a low-level function where we can parse the HTTP response."""
  if resp.status == 200:
    return json.loads(body)
  else:
    raise Exception('unexpected response')

batch = http.BatchHttpRequest(callback, 'https://fcm.googleapis.com/batch')

body = json.dumps({'message': {'topic': 'foo'}})
req = http.HttpRequest(
  http=transport, postproc=postproc,
  uri='https://fcm.googleapis.com/v1/projects/my-project-id/messages:send',
  method='POST',
  body=body)

# Queue 3 requests in the batch
batch.add(req)
batch.add(req)
batch.add(req)

batch.execute()
print(responses) # Prints 3 message IDs