Hide debug-only copy-state command from users

[teor2345]

Motivation

Some zebrad commands are only used for debugging, so it's a bit confusing to show them to users.

We have a few alternatives here:

• disable the copy-state command by default in zebrad release builds using a cargo feature
  • needs Implement a merge queue for Zebra using the bors merge bot #3201 so we can test that the command still builds as part of CI
• hide the copy-state command in the help text
  • needs abscissa 0.6 and clap

Tradeoffs

Adding features creates an ongoing maintenance and testing cost, but it can make builds faster.

Related Work

Follow-up from PR #3175

[teor2345]

This isn't going to happen any time soon.