Create spi extension to forward user delete event to a given URL

Problem/Motivation

From #41 (closed):

In other words, for KC to be able to push a delete event, another small SPI extension would need to be written, maybe even on the same codebase, that consumes the event an triggers an HTTP request somewhere.

Proposed resolution

• Create an SPI extension, ideally on the already existing helpers one, to consume the user delete event.
• Allow configuring a POST request, with a bearer token header as authentication.

Notes

This started at #41 (closed).

It is about reacting on KC user delete, and pushing that into an API call.