net: http_server: serve resources only for their defined services

Ensure that HTTP resources can only be served to a client connected on
the specific service(s) that the resource was registered against using
the HTTP_RESOURCE_DEFINE macro.

This allows different resources to be registered to different services,
for example to make some resources only available via an HTTPS service
and not via unencrypted HTTP.

Signed-off-by: Matt Rodgers <mrodgers@witekio.com>