discover: Handle and track plugin_options

Track plugin_options in the device_handler. Plugins can be added with
device_handler_add_plugin_option() and accessed via
device_handler_get_plugin().
Extend discover_server to support the new 'add' and 'remove' pb-protocol
actions and advertise new plugins to connecting clients.

Signed-off-by: Samuel Mendoza-Jonas <sam@mendozajonas.com>