Support Transcoder in Transaction mode

As custom `Transcoder` is already implemented, and it's fully functional for all non-transactional methods, `TransactionAttemptContext` should use the same configuration.

This heavily impacts the usability for streamlining the codebase, because without this ability, users need to implement own workarounds exclusively for transaction process, even the `Transcoder` is set for cluster.

This feature shouldn't be a complex one, since current codebase already leaves the settings and TODO for using custom Transcoder in transaction-related functions.