Streamline creation of GRPC Exceptions

[daschl]

At the moment, a GRPC exception can be created as

// Without Description
throw new GrpcStatusException(GrpcStatusCode.PERMISSION_DENIED.status());

// With Description
throw new GrpcStatusException(new GrpcStatus(GrpcStatusCode.PERMISSION_DENIED, "my-description"));

Other GRPC implementations provide convenience methods like:

throw PERMISSION_DENIED.withDescription("denied!").asException();

We could potentially add something similar to streamline exception generation (also including withCause, etc).

[cmwigard]

I had a go at this issue as my first contribution to ServiceTalk, I hope you don't mind.
Let me know if there is anything you would like me to change or discuss.