Add UpgradeSequence rpc to 04-channel query service

[damiannolan]

Summary

Return the UpgradeSequence stored in state for the provided port and channel identifiers.

• Add the request and response protobuf message types for querying the upgrade sequence.
• Add the UpgradeSequence rpc to the Query service.
• Implement the query handler in the 04-channel submodule.

rpc UpgradeSequence(QueryUpgradeSequenceRequest) returns (QueryUpgradeSequenceResponse) {
    option (google.api.http).get = "/ibc/core/channel/v1/channels/{channel_id}/ports/{port_id}/upgrade_sequence";
}

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