[TASK] Implement validation over leaf caching for substrate chains

[salman01zp]

Use a local Merkle tree instance in the relayer to validate the leaves against the Merkle root on the chain. You can use the Merkle tree found in: https://github.com/webb-tools/zero-knowledge-gadgets/blob/master/arkworks/native-gadgets/src/merkle_tree.rs.

Each time we add a leaf, it should be imperative to check the state of our local tree against the state on-chain. If it matches, we can proceed. If it doesn't match we need to:

Clear our cache
Start over (fetch all the events using the fast sync for events)
Rebuild the tree
Validate and continue until we get this right.

Evm implementation for reference: #354

[shekohex]

Decreased the priority as this feature is not actually need right now, but it is good to have in the future.