ibc: add support for host chain upgrades

[conorsch]

Some upgrades of the host chain (e.g. Penumbra) might break clients on counterparty chains. To mitigate this we need to implement a loosely specified IBC upgrade mechanism in the same style as ibc-go's UpgradeProposal which, in a nutshell, consists of committing the client and consensus states at a known key.