[SDL 0341] Add Generic HMI Plugin Support

[theresalech]

Proposal: Add Generic HMI Plugin Support

The Generic HMI is currently missing a number of components which are available in the SDL project. The main reason behind this is that several HMI interfaces require data from components of the vehicle beyond SDL (ex. climate module data). This means that many of these vehicle components would need to be emulated in a test environment, and would need to use faked/test data. This makes including these components in the Generic HMI a difficult task, as the project is meant to be a definitive reference implementation for an SDL integration, and introducing faked data to the project would go against that design. Ideally, it should be possible to include the Generic HMI in a production system with minimal modification. This proposal provides a potential solution to this issue by introducing swappable plugins for the Generic HMI. These can be used to easily integrate both test and production implementations of these missing interfaces in the Generic HMI.

Review: smartdevicelink/sdl_evolution#1168