Initial Cut at Graph APIs

[Carter12s]

Description

• Adds a new trait for backends to implement GraphProvider
• Trait Provides get_topics() and get_services() methods that provide basic information about what exists and what their types are
• Provided initial implementations for each backend

New Trait is NOT required for "Ros" trait yet, will eventually fold it in once we're confident in each implementation.

Fixes

Closes: #332

Checklist

• Update CHANGELOG.md