Task: make all signing admin2 commands suitable replacements of corresponding admin commands

[MVrachev]

What is the task about?

We have a new implementation proposed by @lukpueh in his pr #523 that utilizes the latest securesystemslib features.
It's a great start, but we must ensure that this alternative implementation has the same or better user experience, user interface, testing, etc.

This will be an umbrella issue for other smaller issues where the task to review each of those areas will be documented.

TODO

• Task: review & implement missing features of admin2 commands #533
• Task: review & improve if needed UI and UX of admin2 commands #534
• Task: review & improve if needed tests for admin2 commands #535
• Task: review & refactor if needed the code admin2 commands #536

Code of Conduct

• I agree to follow this project's Code of Conduct