GEP: Enable cross namespace references from Routes

[robscott]

What would you like to be added:
This GEP attempts to tackle both cross namespace forwarding and route inclusion (proposed earlier by @stevesloka). These are closely related concepts that can likely be solved with the ReferencePolicy concept originally proposed by @youngnick.

Why is this needed:
Both cross namespace forwarding and route inclusion are common feature requests. ReferencePolicy provides a way to safely accomplish this. This is closely related to #411, #582, and #634.

Relevant docs include:

• Gateway API Reference Policy
• Selection Policy Proposal
• Route Inclusion Proposal
• Cross Namespace Forwarding Proposal

Note: Although the Gateway -> Route relationship is closely related to this discussion, I've decided to tackle that in a later GEP.

[youngnick]

I'll take this one.

/assign