GEP: Refresh Route-Gateway Binding

[robscott]

What would you like to be added:
Refactor cross-namespace Route-Gateway binding to:

• Be more consistent with cross-namespace references from Routes (GEP: Enable cross namespace references from Routes #709)
• Provide a clear path to enable Route inclusion (Routes including Routes).
• Simplify user experience based on initial feedback.
• Enable other kinds of Route parents in addition to Gateway, this could include:
  • Routes (as part of Route inclusion)
  • Custom Gateway resources
  • Mesh resources

Why is this needed:
The existing state is a common source of confusion when interacting with the API and has room for improvement, especially as we're considering adding new capabilities to the API.