Revise SDL-0341 - Add Generic HMI Plugin Support (again)

[jacobkeeler]

Revised proposal based on review comments (see https://github.com/smartdevicelink/sdl_evolution/issues/1168#issuecomment-964475414)

Updates

• Add full definition for configuration file format
• Finalize inter-component message format (along with examples) and include API definitions for all RPCs
• Update wording throughout proposal to be more concrete, finalize several implementation details
• Clarify that all test plugins will be included directly in the repository

[jacobkeeler]

@theresalech Ready for review