Add provisional Enhancement Proposal-2035: Advanced NGINX Extensions (#2039)

Add provisional Enhancement Proposal-2035: Advanced NGINX Extensions

Corresponding Enhancement Proposal issue -- #2035

Co-authored-by: Saylor Berman <[email protected]>

Author: pleshakov

docs/proposals/advanced-nginx-extensions.md

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+# Enhancement Proposal-2035: Advanced NGINX Extensions
+
+- Issue: https://github.com/nginxinc/nginx-gateway-fabric/issues/2035
+- Status: Provisional
+
+## Summary
+
+NGINX Gateway Fabric (NGF) [exposes](/site/content/overview/gateway-api-compatibility.md) NGINX features via Gateway API
+resources (like HTTPRoute) and [NGINX extensions](nginx-extensions.md) (like ClientSettingPolicy). Combined, they
+expose a subset of the most common NGINX configuration. As we implement more Gateway API resources and NGINX extensions,
+the subset will grow. However, it will take time. Additionally, because the number of NGINX configuration directives
+and parameters is huge, not all of them will be supported that way. As a result, users are not able to implement certain
+NGINX use cases. To allow them to implement those use cases, we need to bring a new extension mechanism to NGF.
+
+## Goals
+
+- Allow users to insert NGINX configuration not supported via Gateway API resources or NGINX extensions.
+- Allow users to customize supported NGINX configuration (for example, add a parameter to an NGINX directive).
+- Support configuration from modules not loaded in NGINX by default or third-party modules.
+- Most of the configuration complexity should fall onto the cluster operator persona, not the application developer.
+- Provide security controls to prevent application developers from injecting arbitrary NGINX configuration.
+- Ensure adequate configuration validation to prevent NGINX outages due to invalid configuration.
+- Advanced NGINX extensions can be used without source code modification.
+
+## Non-Goals
+
+- Support configuration other than NGINX directives. For example, njs configuration files or TLS certificates.
+- Reimplement already supported features through the new extension mechanism.