Add Query.get() and MutableData-based transactions to Realtime Database

[pmodernme]

Summary

This PR adds two new features to the Realtime Database module.

Query.get()

Previous implementations used Query.valueEvents.first() to fetch a
value, but that returns either local or remote data depending on
whether persistence is enabled. get() always fetches from the server
and falls back to the locally-cached value (or null) if offline,
giving consistent behavior regardless of persistence settings.
Implemented on Android, Apple, and JS.

runTransaction(MutableData)

A new DatabaseReference.runTransaction overload that mirrors the
Firebase SDK's MutableData-based transaction API. Unlike the existing
serializer-based overload — which always commits — this variant lets
callers explicitly abort a transaction by returning Transaction.abort()
from the handler. Returns the committed DataSnapshot, or null if
aborted.

New public types:

• MutableData — a multiplatform wrapper around the platform
  MutableData type, with key, value, child(), hasChildren,
  and children. Inline value<T>() / setValue<T>() extension
  functions handle serialization.
• Transaction / Transaction.Result — DSL for returning success
  or abort from the transaction handler.

Bug fix

NSNull values in iOS DataSnapshot.value now correctly return
null instead of the NSNull sentinel object.

API surface

Public API binary-compatibility files updated for Android and JVM.

Test plan

• Run the existing firebase-database common tests
• Verify Query.get() returns remote data on Android, iOS, JS
• Verify runTransaction(MutableData) commits and aborts correctly
  on all platforms
• Verify NSNull → null fix on iOS