Allow httpsCallable functions to use the same deserialization as firestore

[ColtonIdle]

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.

[google-oss-bot]

I couldn't figure out how to label this issue, so I've labeled it for a human to triage. Hang tight.

[argzdev]

Hi @ColtonIdle, thanks for this request. With that said, I've marked this ticket so that our engineers can take a look once they have the time.

[ColtonIdle]

@argzdev @schmidt-sebastian has anyone been able to take a look at this? It seems like if documentRef was removed as an arg, then this would be possible?

[thatfiredev]

cc: @ywmei-brt1 who has been doing some work related to serialization.