feat: detach client state verifier enabling users to implement custom verifier

[seanchen1991]

Closes: #979
Closes: #1078

Integration-tests: informalsystems/basecoin-rs#165

Description

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PR author checklist:

• Added changelog entry, using unclog.
• Added tests.
• Linked to GitHub issue.
• Updated code comments and documentation (e.g., docs/).
• Tagged one reviewer who will be the one responsible for shepherding this PR.

Reviewer checklist:

• Reviewed Files changed in the GitHub PR explorer.
• Manually tested (in case integration/unit/mock tests are absent).

[codecov]

Codecov Report

Attention: Patch coverage is 68.16940% with 233 lines in your changes are missing coverage. Please review.

Project coverage is 66.52%. Comparing base (e048e34) to head (5ea1f9b).

Files	Patch %	Lines
...lients/ics07-tendermint/src/client_state/common.rs	19.62%	127 Missing ⚠️
...nts/ics07-tendermint/src/client_state/execution.rs	69.84%	76 Missing ⚠️
...ics07-tendermint/src/client_state/update_client.rs	89.18%	12 Missing ⚠️
.../ics07-tendermint/src/client_state/misbehaviour.rs	90.90%	9 Missing ⚠️
...core/ics24-host/cosmos/src/validate_self_client.rs	0.00%	5 Missing ⚠️
...ts/ics07-tendermint/src/client_state/validation.rs	95.78%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1097      +/-   ##
==========================================
- Coverage   66.53%   66.52%   -0.02%     
==========================================
  Files         204      208       +4     
  Lines       20532    20705     +173     
==========================================
+ Hits        13661    13773     +112     
- Misses       6871     6932      +61     

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[Farhad-Shabani]

We're almost there, just one more step to go! 🙌

The standalone functions currently insist on getting a client_state argument of the ClientState type (like here), making them practically unusable for developers who want to use a custom Tendermint Verifier.
In the custom route, they'd need to define their own newtype wrapper and implement all the ClientState traits on that. Therefore, they should be able to pass their own ClientState type to these functions. Though, there is a simpler way around: If we switch the type of the client_state field to ClientStateType, the problem is solved!

[seanchen1991]

If we switch the type of the client_state field to ClientStateType, the problem is solved

That means we'll need to implement the TmVerifier trait on the ClientState type in the ics07-tendermint-types crate. Is that acceptable?

[Farhad-Shabani]

That means we'll need to implement the TmVerifier trait on the ClientState type in the ics07-tendermint-types crate. Is that acceptable?

Good point! Upon reflection, I realized that we don't necessarily need to implement TmVerifier on a ClientState object. Since it doesn't consume anything from self. Therefore what about defining a unit DefaultVerifier struct and adding an additional argument as verifier: &impl TmVerifier to the verify_header and verify_misbehaviour functions? (Then, we should be able to pass a client state with the type of CientStateType to the standalone functions)
It would look like something as follows:

pub struct DefaultVerifier;

impl TmVerifier for DefaultVerifier {
    type Verifier = ProdVerifier;

    fn verifier(&self) -> Self::Verifier {
        ProdVerifier::default()
    }
}


impl<V> ClientStateValidation<V> for ClientState
where
    V: ClientValidationContext + TmValidationContext,
    V::AnyConsensusState: TryInto<TmConsensusState>,
    ClientError: From<<V::AnyConsensusState as TryInto<TmConsensusState>>::Error>,
{
    fn verify_client_message(
        &self,
        ctx: &V,
        client_id: &ClientId,
        client_message: Any,
    ) -> Result<(), ClientError> {
        verify_client_message(self.inner(), ctx, client_id, client_message, &DefaultVerifier)
    }
}


pub fn verify_client_message<V>(
    client_state: &ClientStateType, // now we should be able to use `CientStateType` here
    ctx: &V,
    client_id: &ClientId,
    client_message: Any,
    verifier: &impl TmVerifier,
) -> Result<(), ClientError>
where
    V: ClientValidationContext + TmValidationContext,
    V::AnyConsensusState: TryInto<TmConsensusState>,
    ClientError: From<<V::AnyConsensusState as TryInto<TmConsensusState>>::Error>,
{
    match client_message.type_url.as_str() {
        TENDERMINT_HEADER_TYPE_URL => {
            let header = TmHeader::try_from(client_message)?;
            verify_header(client_state, ctx, client_id, &header, verifier)
        }
        TENDERMINT_MISBEHAVIOUR_TYPE_URL => {
            let misbehaviour = TmMisbehaviour::try_from(client_message)?;
            verify_misbehaviour(client_state, ctx, client_id, &misbehaviour, verifier)
        }
        _ => Err(ClientError::InvalidUpdateClientMessage),
    }
}

[Farhad-Shabani]

Big thanks for tackling this issue @seanchen1991!
I did some tidying up. Plus, the client_state.rs file was getting pretty massive, making it tricky to move around. While we were pulling out implementations into standalone functions, I figured it was a good opportunity to organize the code better. So, I split the trait implementations into their own modules. Hope that makes sense to you too!