Implement the currency conversion agent with A2A Protocol

Create a Java variant of https://github.com/cmtoan/a2a-samples/tree/main/samples/python/agents/langgraph

I pushed this sample here https://github.com/a2aproject/a2a-samples/pull/434

See the [README](https://github.com/a2aproject/a2a-samples/pull/434/files?short_path=d13d4d0#diff-d13d4d043616d02f4a3271210f4e2a90a024a748710058d99e286a24bbc01383) on how to run it.