Transactions and batch wirtes

KIndly add batch writes and transactions as helpers methods and it will make this library a complete package.

Also exposing collection and document references as getters will be of great help.

Start by reading the library's existing Firebase helper APIs and how collection and document references are currently handled. Define the intended batch-write, transaction, and reference-getter behavior before locating the relevant entry points. Done means the requested helpers are exposed with documented behavior and coverage for their supported operations.

Written by the indexing model from the issue text.