Provisional ProxySettingsPolicy proposal

Author: ciarams87

docs/proposals/proxy-settings.md

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+
+# Enhancement Proposal-4067: Proxy Settings Policy
+
+- Issue: https://github.com/nginx/nginx-gateway-fabric/issues/4067
+- Status: Provisional
+
+## Summary
+
+This Enhancement Proposal introduces the `ProxySettingsPolicy` API that allows Cluster Operators and Application Developers to configure NGINX's proxy buffering and connection settings between the NGINX Gateway Fabric dataplane and upstream applications.
+
+## Goals
+
+- Define proxy settings for buffering configuration.
+- Define an API for proxy settings that is extensible to support additional proxy directives in the future.
+- Outline the attachment points (Gateway and HTTPRoute/GRPCRoute) for the proxy settings policy.
+- Describe the inheritance behavior of proxy settings when multiple policies exist at different levels.
+
+## Non-Goals
+
+- Define the complete set of all proxy directives (only buffering directives are in scope for initial implementation).