[AppCheck] Server side token validation

[abhinavsingh]

Hi, We are looking into using AppCheck for validation of requests received by our server. Currently, AppCheck only seems to support Firebase hosted backends. But in parallel we would like to also perform the same validation (without changing provider) on the server side.

For this to work, we'll need following additions to the platform:

1. Flutter AppCheck API for apps to dispatch attestation token to their server backends (e.g. via GET, POST, WebSocket requests)
2. Firebase Admin SDK API our servers to verify the incoming tokens in the requests.

Thank you!!!