Refactor validated_expected_channel() and check_destination_channel_state()

[joaotav]

Summary

When building channel handshake messages, validated_expected_channel() is used to retrieve the specified channel end from the destination and check if it matches the state that is required for the handshake message to be successful.

validated_expected_channel() and the sub-method check_destination_channel_state() could use some improvement given that:

• validated_expected_channel() currently returns a ChannelEnd object that is never used, it's not clear why this is needed.

• Both the OpenAck and OpenConfirm messages expect the receiving channel end's highest state to be TRYOPEN. It would be helpful to clarify in the comments why this is needed. Is it due to crossing hellos?

// The highest expected state, depends on the message type:
let highest_state = match msg_type {
    ChannelMsgType::OpenAck => State::TryOpen,
    ChannelMsgType::OpenConfirm => State::TryOpen,
    ChannelMsgType::CloseConfirm => State::Open(UpgradeState::NotUpgrading),
    _ => State::Uninitialized,
};

• check_destination_channel_state() could also use refactoring or could be merged into validated_expected_channel()

Proposal

Refactor validated_expected_channel() and check_destination_channel_state() and add comments to enhance clarity. Since this involves the tx CLIs, it can potentially be done together with the work to support multihop connections on those same CLIs (#3940).

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