Add runTransaction(MutableData) to Realtime Database

[pmodernme]

Summary

Adds a 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.

Implemented on Android, Apple, and JS.

Test plan

• Run existing firebase-database common tests
• Verify runTransaction commits correctly on Android, iOS, and JS
• Verify Transaction.abort() returns null on all platforms