Rename Counterparty to ChannelV2

[DimitrisJim]

Raised during aliasing walkthrough.

Rename the Counterparty type into a ChannelV2.

Additional note here is moving this type into a channel/v2 folder.

proto: channel/v2 folder to create new version of channel instead of counterparty in packetserver folder

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[damiannolan]

Seems to be partially handled in #7429

Happy to pick this up for the channel/v2 pkg myself, unless you were planning on it @DimitrisJim? Let me know if its cool! edit: missed dms on slack, happy to leave it to yourself or aditya if its already in progress