[SDL 0294] App Service - Messaging

Proposal: [App Service - Messaging](https://github.com/smartdevicelink/sdl_evolution/blob/master/proposals/0294-app-service-messaging.md)

> This proposal adds a new "messaging" app service for apps that provide messaging features.

Review: https://github.com/smartdevicelink/sdl_evolution/issues/973