TendermintClient::verify_upgrade_and_update_state is not implemented

[romac]

Related to: informalsystems/hermes#722

Summary

The TendermintClient::verify_upgrade_and_update_state is currently not implemented, even though it is called in the UpgradeClient handler.

https://github.com/informalsystems/ibc-rs/blob/efbf7d18512fe6efcb593de68735b7f8783fd652/modules/src/clients/ics07_tendermint/client_def.rs#L395-L403

https://github.com/informalsystems/ibc-rs/blob/efbf7d18512fe6efcb593de68735b7f8783fd652/modules/src/core/ics02_client/handler/upgrade_client.rs#L52-L57

Problem Definition

The UpgradeClient is therefore non-functional at the moment, as it will panic once the method is called.

Proposal

Implement the verify_upgrade_and_update_state method for TendermintClient.

Acceptance Criteria

The verify_upgrade_and_update_state is implemented in TendermintClient, and its behavior is captured in a test for the UpgradeClient handler.

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[plafer]

Until we know how to implement this properly, we should return an Error instead of crashing (either at the tendermint light client level, or even at the core handler level).

Related: #141