shrexnd: requesting proofs only on Client and Server

[Wondertan]

Overview

Shrexnd protocol now only allows requesting data and proofs by namespace, and the goal is to extend the protocol enabling requesting only proofs without data.

Motivation

This is required for the Available method of #1835, which checks whether some data blob is included in the main chain.

Details

The protobuf has to be updated with the new field or new request type and the subsequent implementation of the Server and Client.

Refs

Depends on #1836

[vgonkivs]

Still relevant