REST addon: allow users to implement custom additional routes

[ludolpif]

Hi,
As briefly discussed on FLECS discord, I choosen AGPL on my current fun project (LLSS, a mockup of a OBS-like streaming software, based on game-oriented libraries) and, as it is in early development stages, I want to:

• comply with the licence that explicitly ask to publish source-code through provided network services
• not replace the (minimal and flawky) embed FLECS http server for now

This leads me to try to add a route like "/sources" in the REST API, keeping the others intact. It seems this needs to make a few rest.c functions public, at least: flecs_rest_reply() but flecs_reply_error() and flecs_rest_server_garbage_collect() seems to be very useful also.

Here is my first draft of this. What is done:

• on flecs side: make flecs_rest_reply() public and don't force private_desc.callback = flecs_rest_reply; in flecs_rest_reply()
• a bad and working first example of using this, in example/c/rest

If you still think it could be fine to go for it, could you provide some feedback on this to show me a good way to improve it ?
Thanks,