13/WAKU-STORE: Use multiformat for waku-store digest

[D4nte]

As discussed, in the waku store specs, little information is provided about the digest.
If we do not want to restrict the digest to a specific hash function, then I would suggest to use multihash format.

[staheri14]

Thanks @D4nte for opening the issue and the suggestion!
Thinking that maybe it would be better to enforce a particular hashing function in order to provide consistency across different store nodes. This way, paging indices (cursors) would be reusable from one store node to another.
@oskarth please let me know your thoughts on this.

[staheri14]

In any case, this must be reflected on the specs level to avoid confusion.

[oskarth]

Agree re having a default hashing function explicit. I suppose something like multihash could be used but still generally locked down to a specific hash function? Do we see any specific need for flexibility here? What does Eth2 do?

[staheri14]

Eth2

@oskarth
Looking at eth2 specs, the type of hash function is explicitly stated, Also could not see any use of multihash format
https://github.com/ethereum/eth2.0-specs/blob/dev/specs/phase0/beacon-chain.md#crypto
https://github.com/ethereum/eth2.0-specs/blob/dev/specs/phase0/beacon-chain.md#custom-types