Free atSign API V3 implementation

[athandle]

Is your feature request related to a problem? Please describe.

Consistency in response format endpoint name change

linked to https://github.com/atsign-company/at_handle_registrar_web/issues/415

Describe the solution you'd like

Documentation updated https://docs.google.com/document/d/1X9dbqeB5TE4XIlYzNdenGK1iNAseMs-HdllLF8XPBl8/edit#heading=h.fj6vx9bz6awc

Describe alternatives you've considered

No response

Additional context

URL changes from: /app/v2/login/atsign to: /app/v3/authenticate/atsign

from: /app/v2/login/atsign/validate/ to: /app/v3/authenticate/atsign/activate

from: /api/app/v2/validate-person/ {"data":{"atsigns":["looo","2467wonderful","dsfgdsfdsf","324234234","modestgrecoromanwrestling","bowlinglonely6"],"message":"Oops! You already have the maximum number of free atSigns. Please select one of your existing atSigns. "}} to message moved out of data /api/app/v3/validate-person/ {"data":{"atsigns":["looo","2467wonderful","dsfgdsfdsf","324234234","modestgrecoromanwrestling","bowlinglonely6"]},"message":"Oops! You already have the maximum number of free atSigns. Please select one of your existing atSigns. "}

[sonle-geekyants]

I've changed URL in onboarding on branch feature/change-api-v3 @ayushkumarrojha @sachins-geekyants please test it.

[srieteja]

Changes for Dart registrar API handling have been done as part of https://github.com/atsign-foundation/at_libraries/pull/233.

[srieteja]

Moving into PR49 for Java registrar API handling implementation

[srieteja]

some additional changes from dart api usage reg v2 -> v3 upgrade handled in https://github.com/atsign-foundation/at_libraries/pull/247. Could not work on Java API implementation this sprint, carrying it forward to PR50.