Firebase-admin

- The Admin SDK can now read the Firebase/Google Cloud Platform project ID from both `GCLOUD_PROJECT` and `GOOGLE_CLOUD_PROJECT` environment variables.

Authentication

- The Admin SDK can now create custom tokens without being initialized with service account credentials.
- The SDK now accepts a `serviceAccountId` app option, which can be used to set just the client email of a service account.
- When deployed in an environment managed by Google (e.g. Google App Engine), the SDK can auto discover a service account ID without any explicit configuration.

* [added] A new `auth.import_users()` API for importing users into Firebase Auth in bulk.
* [fixed] The `db.Reference.update()` function now accepts dictionaries with `None` values. This can be used to delete child keys from a reference.

Authentication

- A new [`create_session_cookie()`](https://firebase.google.com/docs/reference/admin/python/firebase_admin.auth#create_session_cookie) method for creating a session cookie from a Firebase ID token.
- A new [`verify_session_cookie()`](https://firebase.google.com/docs/reference/admin/python/firebase_admin.auth#verify_session_cookie) method for validating a given session cookie string.
- The `auth` module now caches the public key certificates used to verify ID tokens and session cookies, thus avoiding a network call everytime a credential needs to be verified.

Cloud Messaging

- Added the `mutable_content` optional field to the [`messaging.Aps`](https://firebase.google.com/docs/reference/admin/python/firebase_admin.messaging#aps) type. This can be used to set the `mutable-content` property when sending FCM messages to APNs targets.
- Added support for specifying arbitrary key-value fields in the `messaging.Aps` type.

Cloud Messaging

- [changed] Improved error handling in FCM by mapping more server-side
errors to client-side error codes. See [documentation](https://firebase.google.com/docs/cloud-messaging/admin/errors).
- [changed] The `messaging` module now supports specifying an HTTP timeout
for all egress requests. Pass the `httpTimeout` option
to `firebase_admin.initialize_app()` before invoking any functions in
`messaging`.

Firebase Cloud Messaging

- A new [`firebase_admin.messaging`](/docs/reference/admin/python/firebase_admin.messaging)
module for sending `messaging` messages and managing topic subscriptions.
- A new [`send()`](/docs/reference/admin/python/firebase_admin.messagingsend)
function for sending `messaging` messages.
- A new [`subscribe_to_topic()`](/docs/reference/admin/python/firebase_admin.messagingsubscribe_to_topic)
function for subscribing a list of device registration tokens to a
`messaging` topic.
- A new [`unsubscribe_from_topic()`](/docs/reference/admin/python/firebase_admin.messagingunsubscribe_from_topic)
function for unsubscribing a list of device registration tokens from a
`messaging` topic.

Firebase Auth

- The [`verify_id_token()`](/docs/reference/admin/python/firebase_admin.authverify_id_token)
function now accepts an optional `check_revoked` parameter. When `True`, an
additional check is performed to see whether the token has been revoked.
- A new [`revoke_refresh_tokens()`](/docs/reference/admin/python/firebase_admin.authrevoke_refresh_tokens)
function to invalidate all tokens issued to a user.
- A new `tokens_valid_after_timestamp` property has been added to the
[`UserRecord`](/docs/reference/admin/python/firebase_admin.authuserrecord)
class to indicate the time before which tokens are not valid.

Initialization

- The [`initialize_app()`](https://firebase.google.com/docs/reference/admin/node/admin#.initializeApp) method can now be invoked without any arguments. This will initialize an app using Google Application Default Credentials, and other options loaded from the `FIREBASE_CONFIG` environment variable.

Database

- The [`db.Reference.get()`](https://firebase.google.com/docs/reference/admin/python/firebase_admin.db#reference) method now accepts an optional `shallow` argument. If set to `True` this will cause the SDK to execute a shallow read, which does not retrieve the child node values of the current reference.