Firebase-admin

* When deployed in the Google App Engine environment, the SDK can now leverage the utilities provided by the [App Engine SDK](https://cloud.google.com/appengine/docs/standard/go/reference) to sign JWT tokens. As a result, it is now possible to initialize the Admin SDK in App Engine without a service account JSON file, and still be able to call [`CustomToken()`](https://godoc.org/firebase.google.com/go/auth#Client.CustomToken) and [`CustomTokenWithClaims()`](https://godoc.org/firebase.google.com/go/auth#Client.CustomTokenWithClaims).

* Now uses the client options provided during [SDK initialization](https://godoc.org/firebase.google.com/go#NewApp) to create the [`http.Client`](https://godoc.org/net/http#Client) that is used to fetch public key certificates. This enables developers to use the ID token verification feature in environments like Google App Engine by providing a platform-specific `http.Client` using [`option.WithHTTPClient()`](https://godoc.org/google.golang.org/api/option#WithHTTPClient).

- [added] Initial release of the Admin .NET SDK. See [Add the Firebase Admin SDK to your Server](https://firebase.google.com/docs/admin/setup/) to get started.
- [added] You can configure the SDK to use service account credentials, user credentials (refresh tokens), or Google Cloud application default credentials to access your Firebase project.

Authentication

- [added] The initial release includes the `CreateCustomTokenAsync()`, and `VerifyIdTokenAsync()` methods for minting custom authentication tokens and verifying Firebase ID tokens.