prop340: Change the ntorv3 extension for negotiation

This tries to see further down the line with our protocol by proposing a ntorv3 extension where the relay version is sent to the Exit in order to inform the endpoint which version the client would like to use.

This allows us to easily flag a circuit for which relay version it needs to be used for.

The only use case for now would packed and fragmented cells at Relay=5 (reserved value) which would allow a tor implementation to know that a specific circuit needs to parse differently bytes on the wire.

Related to #40791

Signed-off-by: David Goulet dgoulet@torproject.org