share: Extend Getter to enable requesting proofs and adjust all the implementations

[Wondertan]

Overview

Current Getter's method GetSharesByNamespace always gets __data and proofs __ by namespace, and the goal is to extend the interface enabling getting 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 recommended way to achieve the goal is to extend GetSharesByNamespace with options defined next to the interface and make each implementation handle them. An alternative could be to make separate GetProofsByNamespace

Refs

Depends on #1837

[distractedm1nd]

Do we really still want this?

[Wondertan]

We have it already as a separate GetProof method. It's required for Rollkit