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 module-info.java.
Note that the onNewClient method will be called when the
RestClientBuilder's build(Class<?> serviceInterface) method is