shrexnd: requesting proofs only on Client and Server

## 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 https://github.com/celestiaorg/celestia-node/issues/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