New OS_API_IMPL, HTTP and REST addons

[SanderMertens]

A new HTTP and REST addon have just been merged with master. A few things to look out for when you update:

• A new FLECS_OS_API_IMPL addon was added to the core with default implementations for Posix/Windows. The addon is enabled by default, which means that you no longer manually need to set the OS API if you want to use threading

• The FLECS_HTTP addon relies on socket & networking APIs. It should work on both Linux and Windows, but so far I've only been able to test it on Linux.

• The FLECS_REST addon implements a REST service that can be used to request entity & query data. To create an application with the REST API enabled, use the following code:

ecs_set(world, 0, EcsRest, {0});

Alternatively, if you use the new FLECS_APP addon, you can enable the REST API like this:

return ecs_app_run(world, &(ecs_app_desc_t) {
    .target_fps = 60,
    .enable_rest = true
});

By default the REST API will listen on port 27750. This can be overridden in the REST component.