Handler for upgrade client

[hu55a1n1]

Crate

ibc

Summary

• handler implementation in ics02_client::handler::upgrade_client.rs
  • raise an issue with cosmos/ics to add canonical algorithm for handling this message? not clear if that's desirable since this is documented and handled mostly in the SDK: https://github.com/cosmos/cosmos-sdk/tree/master/docs/ibc/upgrades
  • probalby inspired from Upgrade Client cosmos-sdk#7367 since there is no canonical algorithm in the cosmos/ics spec (yet)
• clarify the use of upgrade_path field in Tendermint client state and include that in tests

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[plafer]

The upgrade client is not specified in the standard, and we don't have a specific user that needs this. Hence, it's currently unclear how to implement this.

We can get back to it once someone needs it.

[Farhad-Shabani]

As for the upgrade_path relevant context APIs are added in PR #680
Testing these new endpoints should be covered by Basecoin integration tests. For more context see #420 (comment)