RPC layer: support explicit compression of request payloads

Under some circumstances (e.g. bandwidth-constrained deployments, for instance pushing content objects through to a remote objstorage RPC), it would be beneficial to support compression of the payload of requests in our RPC layer.

While compression of request payloads is in theory supported by HTTP, in our tests gunicorn is very unhappy when receiving such a request, and it seems like a very niche feature that few people are using. All in all, a much better bet would be adding explicit control of the compression in our negotiation layer.