Allow httpsCallable functions to use the same deserialization as firestore

## What feature would you like to see?

Allow usage of the typical class mapper in httpsCallables. This feature request came out of this QandA on stackoverflow: https://stackoverflow.com/questions/70448170/firebase-and-android-gethttpscallable-how-to-deserialize-json-into-object/70451194#70451194

## How would you use it?

Essentially, if I have an app that uses purely firestore, then I don't have any typical json deserializer bundled in my app code (because Im using firestore). But if I have a one off call that uses an httpsCallable that returns json, I need to either add a 3rd party library like gson, jackson, etc. I'd ideally just like to use the stuff that is already shipping with my app just for brevity and for the sake that I know how the deserializer behaves with null values, empty values, etc.

No implementation files or tests are named in the issue. Start by locating httpsCallable result deserialization and the Firestore class-mapping path, then compare how each handles returned JSON, null values, and empty values. Done means callable responses can use the existing Firestore mapper without adding Gson, Jackson, or another third-party deserializer.

Written by the indexing model from the issue text.