Firebase-admin

Firebase Auth

- Added the email action link generation APIs for creating links for password reset, email verification and email link sign-in via `auth.PasswordResetLink()`, `auth.EmailVerificationLink()` and `auth.EmailSignInLink()`. Refer [documentation](https://firebase.google.com/docs/auth/admin/email-action-links) for more details and code samples.
- All APIs in the `auth` package now automatically retry RPC calls failing due to I/O or HTTP errors.

- Enabled automatic HTTP retries for the APIs in `db`, `messaging` and `iid` packages.

Firebase Auth

- A new [`SessionCookie()`](https://godoc.org/firebase.google.com/go/auth#Client.SessionCookie) function for creating a [session cookie](https://firebase.google.com/docs/auth/admin/manage-cookies) from a Firebase ID token.
- A new [`VerifySessionCookie()`](https://godoc.org/firebase.google.com/go/auth#Client.VerifySessionCookie) function for validating a given session cookie string.

Firebase Cloud Messaging

- The [`messaging.Aps`](https://godoc.org/firebase.google.com/go/messaging#Aps) type now supports configuring a critical alert sound. A new [`messaging.CriticalSound`](https://godoc.org/firebase.google.com/go/messaging#CriticalSound) type has been introduced for this purpose.
- Public types in the `messaging` package now support correct JSON unmarshalling.

Firebase Auth

- The [`VerifyIDToken()`](https://godoc.org/firebase.google.com/go/auth#Client.VerifyIDToken) function now tolerates a clock skew of up to 5 minutes when comparing JWT timestamps.

- Dropped support for Go 1.8 and earlier.

Cloud Messaging

- `messaging.AndroidNotification` type now supports specifying a `channel_id` when sending messages to Android O devices. Thanks [chemidy](https://github.com/chemidy) for the [contribution](https://github.com/firebase/firebase-admin-go/pull/202).
- `messaging.ApsAlert` type now supports specifying a subtitle. Thanks [chemidy](https://github.com/chemidy) for the [contribution](https://github.com/firebase/firebase-admin-go/pull/191).
- `messaging.WebpushNotification` type now supports specifying additional FCM options. Thanks [chemidy](https://github.com/chemidy) for the [contribution](https://github.com/firebase/firebase-admin-go/pull/185).
- Improved error handling by requesting a more verbose error response from the FCM backend service.

Realtime Database

- Fixed how the SDK handles special characters in node names.

Firebase Database

* [`firebase.App`](https://godoc.org/firebase.google.com/go#App) provides a new `DatabaseWithURL()` function that can be used to initialize clients that connect to different database instances in the same Firebase project.

Authentication

- Fixed a regression introduced in the 3.2.0 release, where [`VerifyIDToken()`](https://godoc.org/firebase.google.com/go/auth#Client.VerifyIDToken) cannot be used in Google App Engine.

Cloud Messaging

- [`messaging.WebpushNotification`](https://godoc.org/firebase.google.com/go/messaging#WebpushNotification) type now supports arbitrary key-value pairs in its payload.