goals and non-goals for session persistence

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
+
+This enhancement proposal extends the Upstream Settings Policy API to support session persistence for both NGINX Plus and NGINX OSS. It enables application developers to configure basic session persistence using `ip_hash` for OSS and cookie-based session persistence for NGINX Plus.
+
+## Goals
+
+- Extend Upstream Settings Policy API to support session persistence.
+
+## Non-Goals
+
+- Provide implementation details for enabling session persistence.
+- Support session persistence for TLSRoute or other Layer 4 routes.
+
+## Introduction
+
+### Extension of Upstream Settings Policy API
+
+
+- explain API
+- How it works for OSS and Plus
+- Describe directives
+- directive contraints
+- sample config
+
+Apps using session persistence must account for aspects like load shedding, draining, and session migration as a part of their application design.
+