input: add input subsystem

Initial commit introducing the input subsystem into Zephyr.

Includes the input_event data structure, the input_report_* APIs, an
iterables sections based subscription API and two operation modes:
synchronous, where the listeners are called directly, and asynchronous,
where the listeners are called in a dedicated thread.

Signed-off-by: Fabio Baltieri <fabiobaltieri@google.com>