at_c: atactivate_registrar interface in packages/atauth

[JeremyTubongbanua]

In packages/atauth, create functions that help interface with the Registrar API. Some HTTP request engineering will need to be done. Ideally, we use MbedTLS for making the HTTP requests.

API Specification

—already got atSign through registrar web— (this is high priority)

• atactivate_registrar_authenticate_person_with_atsign - provide an atSign and request an OTP, after calling this function, an OTP will be sent to the atSign’s corresponding email address
• atactivate_registrar_validate_person_with_atsign_and_otp - provide the OTP (that you got in your email) and you will receive a cram key

—need an atSign through the app— (this is low priority)

• atactivate_registrar_get_free_atsign - fetch and create a new atSign
• atactivate_registrar_register_person - provide atSign and email and it would link them together, and it will send an OTP to your email
• atactivate_registrar_validate_person - provide OTP to the function and it will respond back with a CRAM key