Implementations of this interface will be notified when new Rest Client instances are being constructed. This will
allow implementations to register providers on the RestClientBuilder, and is intended for global providers.
In order for the RestClientBuilder implementation to call implementations of this interface, the implementation must
be specified such that ServiceLoader can find it - i.e. it must be specified in the
file in an archive on the current thread's context classloader's class path - or specified in the
Note that the onNewClient method will be called when the RestClientBuilder's
build(Class<?> serviceInterface) method is called.