[firebase_crashlytics] setCustomKeysAndValues

[diegog-sf]

I would like to propose exposing the crashlytics function setCustomKeysAndValues (ios) or setCustomKeys (android).

The plugin currently has a call to register a single value for a key (setCustomKey), but on our app, we register ~50 values at the same time, so we could really use this function to register them all at the same time.

Native docs
ios