lib: Add plugin_option type and protocol

Add a new struct 'plugin_option' to represent pb-plugins that are
installed on the system. This consists of plugin metadata and an array
of installed executables.
This also adds two new pb-protocol actions to advertise the addition of
a new plugin_option, and to remove known plugin_options.

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