FR: App Check Token Validation

* Operating System version: N/A
* Firebase SDK version: N/A
* Library version: N/A
* Firebase Product: App Check

Currently, App Check validation is only available with the NodeJS-based Admin SDK. This limits the ability to [Verify App Check tokens from a custom backend](https://firebase.google.com/docs/app-check/custom-resource-backend) to Node-compatible applications.

Go is a very popular language for writing custom backends. It would be helpful if the Firebase Admin Go SDK provided similar functionality to the NodeJS SDK for Go backend developers to verify app check tokens.

Off the cuff, I would find an interface like this helpful (inspired by the NodeJS interface):

```golang
client, err := app.AppCheck(ctx)

claims, err := client.VerifyToken(token)
```