Support IBC Client Params

[rnbguy]

Feature Summary

Right now, ibc-rs doesn't support IBC Client params. With Client params, the host chain can disable or enable supported client types via governance.

Proposal

Let me differentiate between supported clients and allowed clients.

• Supported Clients: This is implementation specific. This indicates the client state decoders available in the host chain implementation.
• Allowed Clients: This is blockchain specific. The blockchain community decides which client state decoders should be allowed on the chain via governance.

The allowed clients must also be supported. Otherwise, the implementation can't decode a valid allowed client.

IMO, the distinction is required to give flexibility to the community without any developer intervention. This is useful in the case of any faulty implementation of a client type. A chain can immediately pass a governance to disable a client type without a chain upgrade.

Implementation

I propose to use ClientState macro to create a method called supported_clients to return a set of the supported client types corresponding to the enum variants.

At Genesis

In the genesis, the client params can have a subset of the supported client types as allowed clients. And the subset condition is checked at ibc module initialization.

At Client Create or Client Upgrade

Successfully create or upgrade client state if the client state can be decoded to one of the allowed_clients types - whose decoders are available. Otherwise, the transaction fails.

At Client Params Update

Successfully update the client params on the blockchain if the provided allowed_clients is a subset of supported_clients. Otherwise, the transaction fails.

[rnbguy]

This feature will also complete the unimplemented ClientParams query in #842.