Add ability for plugin-pipeline external, persistent processes

[cfirthae]

OnAIR should have the room to add extra capabilities that operate outside of the plugin sense-perceive-act loop, but persist so as to enable all kinds of operations not constrained by SPA: data storage across different loops required by multiple plugins, logging, file handoffs, etc. A tentative name for these processes would be "Services", a la similar functions in pub/sub paradigms, but in OnAIR they will necessarily be lighter weight and not require an underlying communication bus. Instead, instantiations of this idea should make these processes directly available to plugins.