Switch to domain event type for the upgrade_client_proposal_handler return

[Farhad-Shabani]

Problem Statement

The upgrade_client_proposal_handler function currently returns an event type from the tendermint_proto which starting from tendermint-rs v0.33.0 will be only compatible with CometBFT v0.38.
The tendermint-proto now provides support for multiple versions of CometBFT. So to ensure the handler compatibility with different versions and avoid making assumptions about hosts, the return type should be modified to tendermint::abci::Event. This allows for easy conversion to any version of proto events, allowing for compatibility with different hosts.