Allow overriding the host for secure token requests

### Feature proposal

* Firebase Component: Auth

Please allow overriding the host for secure token requests to enable developers using their own reverse proxy in places where securetoken.googleapis.com is blocked.

There is currently a [private API on FIRSecureTokenRequest](https://github.com/firebase/firebase-ios-sdk/blob/e05d9111eebb5ced9021d2d5aebb18a2db5f8b18/Firebase/Auth/Source/Backend/RPC/FIRSecureTokenRequest.m#L157-L159) to set the host. It should be sufficient to expose this method in the header.

Overriding the host is already possible for other Firebase services (e.g. Firestore as explained [here](https://github.com/firebase/firebase-ios-sdk/issues/1300#issuecomment-541458399)).