Design Session Persistence - Part 1 (#4158)

Problem: Need to get consensus on goals and non-goals for session persistence.

Solution: Create a proposal doc to outline goals and non goals of the feature

Author: salonichf5

docs/proposals/session-persistence.md

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+# Enhancement Proposal-4051: Session Persistence for NGINX Plus and OSS
+
+- Issue: https://github.com/nginx/nginx-gateway-fabric/issues/4051
+- Status: Provisional
+
+## Summary
+
+Enable NGINX Gateway Fabric to support session persistence for both NGINX Plus and NGINX OSS, allowing application developers to configure basic session persistence using the `ip_hash` load balancing method in OSS and cookie-based session persistence in NGINX Plus.
+
+## Goals
+
+- Extend the Upstream Settings Policy API to allow specifying `ip_hash` load balancing method to support basic session persistence.
+- Design the translation of the Gateway API `sessionPersistence` specification, which can be configured on both HTTPRoute and GRPCRoute, into NGINX Plus cookie-based session persistence directives with `secure` and `httpOnly` mode enforced by default.
+
+## Non-Goals
+
+- Describe or implement low-level configuration details for enabling session persistence.
+- Extend session persistence support to TLSRoutes or other Layer 4 route types.
+- Supporting the `sameSite` cookie directive for NGINX Plus session persistence, which may be considered in the future as the Gateway API `sessionPersistence` specification evolves.
+
+## Useful Links
+
+- Session Persistence [specification](https://gateway-api.sigs.k8s.io/reference/spec/#sessionpersistence).
+- Extended Session Persistence [GEP](https://gateway-api.sigs.k8s.io/geps/gep-1619).
+- RFC standard for [Set-Cookie](https://datatracker.ietf.org/doc/html/rfc6265#section-4.1) header.