Firebase-admin

Firebase Auth

- [Fixed] User management APIs now correctly raise `ErrorCode.AlreadyExists` when an already in-use email address is specified for a user. Thanks [kcieslinski](https://github.com/kcieslinski) for the [contribution](https://github.com/firebase/firebase-admin-dotnet/pull/141).

- SDK automatically retries HTTP requests that are failing due to low-level I/O errors and HTTP 503 errors.

Firebase Auth

- [`ExportedUserRecord`](https://firebase.google.com/docs/reference/admin/dotnet/class/firebase-admin/auth/exported-user-record) does not expose password hashes that are redacted due to lack of permissions in the service account credentials.

- Fixed an issue with IAM-based custom token signing, that resulted in permission errors.

- Fixed an issue that caused async API calls to deadlock in some environments. Thanks [NiCoTinEz](https://github.com/NiCoTinEz) for the [contribution](https://github.com/firebase/firebase-admin-dotnet/pull/109).

Firebase Auth
- Introduced a new `FirebaseAuthException` class and an `AuthErrorCode` enum type for improved error handling support. All user management APIs now throw instances of the `FirebaseAuthException` class. This is a subclass of the existing `FirebaseException` type.
- ID token verification APIs now throw instances of the `FirebaseAuthException` class.
- `CreateCustomTokenAsync()` APIs now throw `FirebaseAuthException` if an error occurs while signing custom tokens. If the service account is not correctly configured, this API throws an `InvalidOperationException`.

Cloud Messaging

- Added `SubscribeToTopicAsync()` and `UnsubscribeFromTopicAsync()` methods to the [`FirebaseMessaging`](https://firebase.google.com/docs/reference/admin/dotnet/class/firebase-admin/messaging/firebase-messaging.html) class for managing topic subscriptions. Thanks [Leo-Mepham](https://github.com/Leo-Mepham) for the [contribution](https://github.com/firebase/firebase-admin-dotnet/pull/94).
- Added support for specifying the analytics label for notifications via [`AndroidFcmOptions`](https://firebase.google.com/docs/reference/admin/dotnet/class/firebase-admin/messaging/android-fcm-options.html), [`ApnsFcmOptions`](https://firebase.google.com/docs/reference/admin/dotnet/class/firebase-admin/messaging/apns-fcm-options.html) and [`FcmOptions`](https://firebase.google.com/docs/reference/admin/dotnet/class/firebase-admin/messaging/fcm-options.html) classes. Thanks [RannyRanny](https://github.com/RannyRanny) for the [contribution](https://github.com/firebase/firebase-admin-dotnet/pull/89).
- Introduced a new [`FirebaseMessagingException`](https://firebase.google.com/docs/reference/admin/dotnet/class/firebase-admin/messaging/firebase-messaging-exception.html) class and a [`MessagingErrorCode`](https://firebase.google.com/docs/reference/admin/dotnet/namespace/firebase-admin/messaging.html#namespace_firebase_admin_1_1_messaging_1ac3482b9d4581255dee08b81ac3f677c9) enum type for improved error handling support. All FCM APIs now throw instances of the `FirebaseMessagingException` class. This is a subclass of the existing `FirebaseException` type.

- Added `netstandard2.0` to the `TargetFrameworks` list of the package.
- Began compiling releases on the .NET Framework instead of Mono. This should resolve some of the library linkage issues reported by developers on previous versions of the SDK.

Firebase Auth

- Added the [`CreateUserAsync()`](https://firebase.google.com/docs/reference/admin/dotnet/class/firebase-admin/auth/firebase-auth.html#createuserasync) method for creating new user accounts.
- Added the [`UpdateUserAsync()`](https://firebase.google.com/docs/reference/admin/dotnet/class/firebase-admin/auth/firebase-auth.html#updateuserasync) method for updating exiting user accounts.
- Added the [`ListUsersAsync()`](https://firebase.google.com/docs/reference/admin/dotnet/class/firebase-admin/auth/firebase-auth.html#listusersasync) method for listing or iterating over all users accounts in a Firebase project. Thanks [ChristopherLenz](https://github.com/ChristopherLenz) for the [contribution](https://github.com/firebase/firebase-admin-dotnet/pull/77).

Auth

- Added [`GetUserByEmailAsync()`](https://firebase.google.com/docs/reference/admin/dotnet/class/firebase-admin/auth/firebase-auth#getuserasync) and [`GetUserByPhoneNumberAsync()`](https://firebase.google.com/docs/reference/admin/dotnet/class/firebase-admin/auth/firebase-auth#getuserasync) methods for retrieving user accounts.

Cloud Messaging

- Added [`WebpushFcmOptions`](https://firebase.google.com/docs/reference/admin/dotnet/class/firebase-admin/messaging/webpush-fcm-options) API for specifying Webpush-specific notification options. Thanks [Odonno](https://github.com/Odonno) for the [contribution](https://github.com/firebase/firebase-admin-dotnet/pull/64).