Firebase-admin

Firebase Auth

- Added a new [`GetUserAsync()`](https://firebase.google.com/docs/reference/admin/dotnet/class/firebase-admin/auth/firebase-auth#getuserasync) method that allows retrieving user accounts.
- Added a new [`DeleteUserAsync()`](https://firebase.google.com/docs/reference/admin/dotnet/class/firebase-admin/auth/firebase-auth#deleteuserasync) method that allows deleting user accounts.

- [added] `AppOptions` class now supports setting an `HttpClientFactory`, which is useful when deploying the SDK behind a proxy server.

Firebase Cloud Messaging

- Added a new [`SendMulticastAsync()`](https://firebase.google.com/docs/reference/admin/dotnet/class/firebase-admin/messaging/firebase-messaging#sendmulticastasync) API for sending a message to a list of device registration tokens. Thanks [kentcb](https://github.com/kentcb) for the [contribution](https://github.com/firebase/firebase-admin-dotnet/pull/36).
- Added a new [`SendAllAsync()`](https://firebase.google.com/docs/reference/admin/dotnet/class/firebase-admin/messaging/firebase-messaging#sendallasync) API for sending a list of messages as a single batch. Thanks [kentcb](https://github.com/kentcb) for the [contribution](https://github.com/firebase/firebase-admin-dotnet/pull/36).

Auth

- The `VerifyIdTokenAsync()` method now tolerates a clock skew of up to 5 minutes when comparing JWT timestamps.

- A new [`FirebaseMessaging`](https://firebase.google.com/docs/reference/admin/dotnet/class/firebase-admin/messaging/firebase-messaging) class for sending Firebase Cloud Messaging messages.

Authentication

* Implemented the `SetCustomUserClaimsAsync()` API in the `FirebaseAuth` class.