Partially implement IBC client update of sovereign rollups on Cosmos chains

[soareschen]

Part of: #10
Prerequisites: #97
also informalsystems/sovereign-ibc#65

Summary

This is similar to #97, except that we are partially implementing IBC client update.

• The UpdateClient message would contain a valid Celestia DA client update header.
• The rollup client update would be mocked, i.e. the final state hash is randomly generated.
  • However, the initial rollup state hash should match the IBC client current rollup state hash
  • We will still not be communicating with the rollup node in this PR.
• The UpdateClient message would contain a dummy aggregate proof, i.e. randomly generated blob.

Implementation strategy

Similar to #97, we would implement context-generic update client components as something like:

pub struct UpdateSovereignClientOnCosmos;

impl<SovereignChain, CosmosChain>
    ProvideUpdateClientPayloadType<SovereignChain, CosmosChain> 
    for UpdateSovereignClientOnCosmos
{ /* ... */ }

impl<SovereignChain, CosmosChain>
    UpdateClientPayloadBuilder<SovereignChain, CosmosChain> 
    for UpdateSovereignClientOnCosmos
{ /* ... */ }

impl<CosmosChain, SovereignChain>
    UpdateClientMessageBuilder<CosmosChain, SovereignChain>
    for UpdateSovereignClientOnCosmos
{ /* ... */ }

When testing, we would mock the client ID on the sovereign rollup so that we can perform the IBC client creation on the sovereign rollup at a later time. The SovereignToCosmosRelay context should implement CanBuildUpdateClientMessage via BuildUpdateClientMessages, with all the depedencies implemented. The test would expand on the original create client test and does something like:

#[test]
async fn test_sovereign_to_cosmos_create_client() -> Result<(), Error> {
    let cosmos_bootstrap = CosmosBootstrap { /* ... */ };

    let sovereign_bootstrap = SovereignBootstrap { /* ... */ };

    let cosmos_chain = cosmos_bootstrap.bootstrap_chain().await?;

    let sovereign_chain = sovereign_bootstrap.bootstrap_chain().await?;

    let create_client_options = SovereignToCosmosCreateClientOptions { /* ... */ };

    let cosmos_client_id = SovereignToCosmosRelay::create_client(
        DesitinationTarget,
        &cosmos_chain,
        &sovereign_chain,
        &create_client_options,
    ).await?;

    let sovereign_client_id = ClientId::default(); //mock

    let sovereign_to_cosmos_relay = SovereignToCosmosRelay { /* ... */ };

    let update_height = sovereign_chain.query_chain_height()?;

    let update_client_messages = sovereign_to_cosmos_relay
        .build_update_client_messages(
            DesitinationTarget,
            update_height,
        )?;
    
    cosmos_chain.send_messages(update_client_messages).await?;

    Ok(())
}