es_out: move private control to input-specific interfaces

es_out_t provides a priv_control callback which is only exposed and used by the es_out_t implementations provided by the input (es_out_source, es_out_timeshift and the main es_out implementation from the input).

This merge request moves this callback to an internal structure exposed in src/input/es_out.h, as well as providing a common virtual table for potential future extensions.