Add MR discussion pagination

Similar to !433 (merged), starting from %15.3 we need to paginate MR discussions. The issue is to add this support.

Explore if caching can be emulated as @patrickbajao explained:

We also introduced HTTP caching to discussions.json endpoint (this exists for both issues and MRs). I'm thinking that if a response header has etag on it, on succeeding request the If-None-Match header can be set with the value of the etag from the previous request's response. Is it possible to do that?

See gitlab-org/gitlab#342612 (comment 1059554169) for more detailed information.