Add a v6 transaction format with no changes

[mpguerra]

Motivation

A number of teams are working on the v6 transaction format for NU6 and they are looking to add this to Zebra.

The format should initially be the same as v5 transactions, and only valid when NU6 is activated.

[mpguerra]

Hey team! Please add your planning poker estimate with Zenhub @arya2 @oxarbitrage @teor2345 @upbqdn

[upbqdn]

Is the scope of this ticket limited to only adding a v6 enum variant to the Transaction enum without adding any further logic?

[teor2345]

Is the scope of this ticket limited to only adding a v6 enum variant to the Transaction enum without adding any further logic?

It's important that v6 is only valid when NU6 is activated, otherwise we'd be breaking the NU5 consensus rules.

As another alternative, we could put all this code behind a feature.