Added minimal support for sending FCM messages in async using HTTP/2

[jonathanedey]

Adds basic logic for sending FCM messages in async using HTTP/2

This initial PR covers:

• Exporting async method send_each_async()
• Handling refreshing of google.auth credentials and retrying refresh on 401 errors
• Basic retry logic for 503 and 500 errors (no backoff)
• Status and request error mapping to FirebaseErrors
• Simple unit and integration tests

[lahirumaramba]

As we discussed offline, let's confirm if this complex code path is necessary for error handling. If it is not, let's clean it up and keep it simple :)

[jonathanedey]

Looks like we use this map FCM specific errors like:

FCM_ERROR_TYPES = {
        'APNS_AUTH_ERROR': ThirdPartyAuthError,
        'QUOTA_EXCEEDED': QuotaExceededError,
        'SENDER_ID_MISMATCH': SenderIdMismatchError,
        'THIRD_PARTY_AUTH_ERROR': ThirdPartyAuthError,
        'UNREGISTERED': UnregisteredError,
}

We map to these errors by parsing the error response details. The default handle_func (handle_X_error) is agnostic to the source service of the error and doesn't try to parse based the error response details. FCM is the only service that currently makes use of this optional handle_func but this seems like the correct way to support this scenario.

We could move the logic from the helper function _handle_func_X to handle_platform_error_from_X.

Will merge as is and we can revisit this in a follow up PR if necessary.

[lahirumaramba]

let's add this new dependency to setup.py