Add (incoming) Webhook model #86

When we receive a webhook string (json) to our url, we would like to parse the json into a IncomingWebHook.java.
The open api currently doesn't seem to have webhooks or callback defined. i.e. https://learn.openapis.org/examples/v3.1/webhook-example.html

The java library would benefit from having the model for webhook fireblocks sends.

Secondly, the library could have webhook signature verificator. i.e. check the fireblocks public certificate and validate SHA512 signature agains the string.

public IncomingWebHook parseIncomingWebHook(String json, String signature) {
  ...
}