use built-in Javascript API for REST API calls

[mscurtescu]

Describe the bug

REST API used to get authentication request URL for the "Continue with Hellō" button must use WordPress specific JavaScript APIs so that proper authentication is sent to the API.

Currently the API implementation is not depending on authenticated user, but that can change.

See: https://developer.wordpress.org/rest-api/using-the-rest-api/authentication/

For developers using the built-in Javascript API, this is handled automatically for you. This is the recommended way to use the API for plugins and themes. Custom data models can extend wp.api.models.Base to ensure this is sent correctly for any custom requests.

• https://developer.wordpress.org/rest-api/using-the-rest-api/backbone-javascript-client/

[mscurtescu]

no relevant anymore, Quickstart moved to regular endpoint

[mscurtescu]

still relevant for now, one more REST API