Add transaction signing example code in java

## Why

To make the repo have e2e testing and cover all the test cases, transaction signing should also be included.
## What

We need to have a java based transaction signing code example similar to what we have in ts - https://github.com/cardano-foundation/cardano-rosetta/blob/master/examples/postman/sign-transaction-server.js

Start by reading the referenced examples/postman/sign-transaction-server.js implementation to understand the expected transaction-signing flow. Identify the corresponding Java example entry point in this repository, then add equivalent signing coverage and verify that the end-to-end test cases include transaction signing.

Written by the indexing model from the issue text.