Allow for setting in custom API_DOMAIN via the constructor

[netvarun]

It would be great if we could do something like this:

String apiKey = "SEM3xxxxxxxxxxxxxxxxxxxxxx";
String apiSecret = "xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx";
String endpoint = "webhooks";
String apiDomain = "api-test.semantics3.com";

Semantics3Request request = new Semantics3Request(apiKey, apiSecret, endpoint, apiDomain);

btw, this is supported in the other libraries - Perl, Python, Node, Ruby