Experiment: Transaction::V6 variants only

[aphelionz]

Motivation + Solution

On a recent call with @conradoplg and @arya2, we decided to try this out to make the NSM and ZSA PRs more managable.

This PR only adds the Transaction::V6 enum variant and then satisfies the compiler. The variant and its respective match arms and array members are feature-flagged under tx_v6.

Tests

• cargo test --workspace locally

Follow-up Work

If this approach is suitable, I can do NetworkUpgrade::Nu7 as well.

PR Checklist

• The PR name is suitable for the release notes.
• The solution is tested.
• The documentation is up to date.
• The PR has a priority label.
• If the PR shouldn't be in the release notes, it has the
  C-exclude-from-changelog label.