state fraud proofs (celestia): verification logic

[liamsi]

Verifying a fraud proof requires knowledge of the state logic. We need to investigate which parts will have to live:

1. in the abci app itself
2. in the Cosmos-SDK
3. lazyledger-core

Lazyledger-core most likely should provide a simple library (reactor?) to gossip fraud proofs. Maybe the evidence pool can be re-used for that. This should be as light-weight as possible, as we would want light-clients to also participate in the gossiping.
Fraud proofs will be gossiped by Celestia-nodes: https:/celestiaorg/celestia-node
Additionally, celestia-core needs to collect the intermediate state in Block.Data (celestiaorg/celestia-core#84).

The Cosmos-SDK probably needs modifications to produce these intermediate state roots (celestiaorg/cosmos-sdk#8 and celestiaorg/cosmos-sdk#9).

Fraud proofs generation and verification could happen in a separate module/in the lazyledger-app itself.

The overall design should be part of rollkit/rollkit#23 too. ADRs that link to each other in the particular repositories will inform this design document.

Ideally, the design should be independent of which tree is actually used (iavl/smt/other) (ref: celestiaorg/cosmos-sdk#8)

[Wondertan]

@liamsi, looks like the description needs to be updated according to the latest plans

[liamsi]

Thanks for the nudge. Updated the gossiping part.