feat(share/p2p/shrexnd): Non-Inclusion Proofs do not exist for ErrNamespaceNotFound

[distractedm1nd]

In #2145 , we will introduce a new response type for the case when data exists, but the namespace requested is not found in the data. This introduces a critical attack vector where FNs can tell LNs that data does not exist for a specific nID at arbitrary heights.

The solution for this is implementing non-inclusion proofs. The signature for GetSharesByNamespace does not need to change, as it fits inside the nmt.Proof data structure.