🚀 Feature: Ability to add user via backend rest api

[fallenleavesgocrunch]

Feature description

The ability for a backend application to create pocket-id users through a REST API.

Pitch

I'd also love to integrate pocket-id in to software I'm writing; but for that to work I cannot manually administer adding users. I'd love it if there were some kind of REST api where I can submit a new user to pocket-id and have it send out the authenticate email with one-time setup link and a redirect back to my app when it's setup.

app.backend -> pocket-id.backend/add-user?username=xxxx?group=zzzzz&returnto=urlurlurl

pocket-id would then create the user, add them to the group, and email them out the one-time invite. When the user has authenticated their email and added a passkey it would then redirect them to the returnto url.