Add "signup" / create user in authentication-jwt module

[dhmlau]

Suggestion

In the authentication-jwt extension, the user experience would be much nicer if it includes the default implementation for creating user as well.

I'm trying to use this extension in my app, and I'd also want to have the "signup" REST API. In order to do that, I need to (I'm still in the middle of it, so the list may not be complete):

• add the signup endpoint in the UserController

cc @jannyHou @raymondfeng

Acceptance criteria

• Add the signup demo in the controller test fixture

[jannyHou]

@dhmlau Agreed 👍

[dhmlau]

For your reference, i've added the signup endpoint in my repo: https://github.com/dhmlau/loopback4-example-todo-jwt/.

[dhmlau]

Resolved via #5763. Closing as done.