Internal Data Model Design and Implementation

[danehans]

#16 specifies the need for an Intermediate Representation (IR) component to decouple the system from Gateway API. The IR details should be captured in this issue or a linked doc.

[skriss]

For reference, Contour's internal data model ("the DAG"), which all APIs (Gateway API, Ingress, HTTPProxy) are converted to before emitting Envoy config, is defined here: https://github.com/projectcontour/contour/blob/main/internal/dag/dag.go