Implement async fetch functionality in the TeaVm Fruzhin project.

[Zurcusa]

In order to implement #494 and have decent performance we need the possibility to send async requests.

Find a way to implement that functionality (most probably via a fetch request). Thins to consider:

• Having a reusable representation (in the form of an object) for RPCRequests and RPCResponses.
• Having access to the response data in the Java code. Possible options might be using callbacks or the TeaVM Promise implementation JSPromise.