Adds the ability to specify a client_id as part of a SignatureRequest

While we provide a way to set a client_id on the EmbeddedRequest object, we did not have a way to specify the client_id for non-embedded requests (e.g., for white labeling).

This update adds the ability to set the client_id on a SignatureRequest to take advantage of API App configurations for non-embedded requests:

SignatureRequest req = new SignatureRequest();
req.setClientId("YOUR_API_APP_CLIENT_ID_HERE");
HelloSignClient client = new HelloSignClient(auth);
client.sendSignatureRequest(req);