Add bindings for metrics functions

[gavv]

Follow-up for #118.

We need to add bindings for C functions roc_sender_query() (becomes Sender.Query() in Go) and roc_receiver_query() (becomes Receiver.Query() in Go).

New functions should be covered with simple tests (errors path and OK path).

Documentation for C API can be found here: https://roc-streaming.org/toolkit/docs/api/reference.html

C headers are here: sender.h, receiver.h.

[lerrxws]

Hi @gavv! I'd like to give this one a try if that's okay.