feat: introduce a new 'register' package with consolidated code

[srieteja]

Closes #468

- What I did

• Consolidated code from Register_cli and onboarding_flutter, pertaining to fetching free atsigns and getting them registered to an email.
• This new consolidated code has been written into a package which is now called AtRegister.
• Consume AtRegister in register_cli and make necessary changes to allow this consumption

- How I did it

- How to verify it

• Unit tests present in at_register package
• Unit tests present in onboarding_cli to validate register_cli which now consumes AtRegister

- Description for the changelog

feat: introduce a new 'AtRegister' package with consolidated code reg registration of free atsigns

[gkc]

@srieteja @sitaram-kalluri This PR is very old. Should we close it, and re-prioritize #468 ? There was a lot of work here, it's not clear to me why it was abandoned

[srieteja]

@gkc, this is a part of #468. We decided to take focus off apps for some time and this one fell through the cracks. The work here is still relevant to upgrade onboarding_cli from using the registrar v2 API to v3 API. I can put some time into it next week and bring this up to speed if we believe this is a priority.

[gkc]

@srieteja You should stay focused on the C impl of at_activate. Please mark this as draft and add a comment to the associated ticket.

[srieteja]

@srieteja You should stay focused on the C impl of at_activate. Please mark this as draft and add a comment to the associated ticket.

Sure @gkc