Add or extend a relation to provide details about the gateway

[ca-scribner]

Enhancement Proposal

As discussed here, some use cases for the gateway require details about the Gateway so they can create routes themselves. We should add this capability, either by using the existing ingress relation or adding another.

An example approach for this is the istio-pilot gateway-info library which solved essentially this problem for istio-pilot. I think for the Gateway API we need slightly richer data, but generally the idea is the same

[lucabello]

We should implement the existing relation interface.