share: Extend `GetSharesByNamespace` with an option to collect Merkle proofs

[Wondertan]

Context

We want FN/BNs to serve data by namespace as a whole. For that, we need to read Merkle proofs while getting data by namespace so that requestors can verify its inclusion in the header.

Implementation Details

• share.GetSharesByNamespace is used to load data from disk only and not from the network anymore
  • Using Blockservice with offline exchange
  • Using Blockstore provided by EDSStore
• share.GetSharesByNamespace is extended to return NMT Merkle proofs
  • Similar to share/ipld.GetProofsForShares
  • Ensure Merkle proofs are not duplicated!

Refs

Depends on #878 Depends on #183