Move Provider IR Fields to Emitter IR

[danehans]

Based on recent upstream changes, Ingress NGINX-related provider IR fields should be moved to the emitter IR.

• See Emitters Implementation kubernetes-sigs/ingress2gateway#273 for upstream emitter implementation details.
• Feature/path rewrite kubernetes-sigs/ingress2gateway#288 is an example of an Ingress NGINX provider feature that adds fields to the emitter IR.
• Update agentgateway and kgateway emitters as needed.
• Update emitter and provider documentation (README.md) as needed.