[SDL 0224] Navigation Subscription Buttons

[theresalech]

Proposal: Navigation Subscription Buttons

This proposal adds support for new subscription buttons (like the existing Play/Pause, Next, Previous, etc.) specifically designed for navigation scenarios.

Review: smartdevicelink/sdl_evolution#712

Steering Committee Decision:

The Steering Committee voted to accept this proposal with the following revisions:

• Incorporate this comment, with NAV_ROTATE including rotating left or right 90 degrees.
• Add button press to the Navigation Service's handled RPCs with these new button types.

Additionally, there was a question in the Steering Committee meeting about requiring applications to be compatible with this proposal. It was advised that this requirement is out of the scope of SDL, and OEMs would need to ask application developers to implement this proposal once it's available in the library for developers to implement.

The proposal .md file was updated to reflect these revisions on 5/20/19.