Return proof hash on deposits

[mfornet]

For the frontend it is useful to filter txs by proofs. But proofs usually are very large so indexer tools struggle to allow fast filtering under this key. What we can do on the smart contract side is find the hash of the proof (sha256(rawProof)), and when a tx is successful and the proof is consumed, return this hash which will be included in the receipt.

This will allow filtering fast through the proofs from regular indexer tools.

Note: beware that the result of this receipt can't be used to prove unexpected information on the other side of the bridge.

[mfornet]

There are some problems with this proposal. The transaction is recorded on in the middle of a sequence of cross contract calls, so it is not possible to return the proposed value at this point (so it becomes a receipt).

Alternatives:

1. Make a parallel cross-contract call to self (in the factory) after the proof is recorded, with the hash of the proof so a receipt is emitted.
2. Emit a log with the hash after the cross-contract call.

1 is far from ideal, since it incurs in more gas consumption, with is complex enough with regard to the number of cross-contract calls that are made in a deposit txs.

2 is less invasive, the problem is that the current indexer doesn't store logs, so it is impossible to search/filter through them.

My proposal here is to use option 2, and improve the tooling locally, such that it is possible to filter by logs.