docs: add provisional GEP for Gateway Firewall Support

Signed-off-by: Shane Utt <[email protected]>

Author: shaneutt

geps/gep-3614/index.md

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+# GEP-3614: Firewall
+
+* Issue: [#3614](https://github.com/kubernetes-sigs/gateway-api/issues/3614)
+* Status: Provisional
+
+## TLDR
+
+The ability to attach firewall rules for ingress L3, L4 and L7 Gateway traffic.
+
+## Motivation
+
+`Gateways` are commonly exposed to the internet, which puts them as risk of
+attack. Internal networks can become compromised as well. We should provide
+tooling, documentation and best-practices for users to restrict and control
+access to their `Gateways`.
+
+### User Stories
+
+* As an application developer, I want to allow specific IPs to access my
+  application.
+* As an application developer, I want all ingress traffic for my Gateways to be
+  restricted to my CDN.
+* As an application developer, I want to block all requests from specific user
+  agents OR only allow specific user agents.
+* As a gateway operator I want to be able to identify and block malformed HTTP
+  requests before they reach backend applications.
+* As a gateway operator I want to be able to provide my own signature-based
+  detection rulesets to spot patterns of known malicious traffic and block
+  them, updating those rules dynamically over time.
+* As a gateway operator I want to attach complete rulesets maintained by
+  upstream standards bodies to block well known common threats and dynamically
+  update for new threats over time.
+* As a cluster operator I want to be able to block traffic to gateways from
+  specific geographical regions, or only allow specific regions.
+
+## Goals
+
+* Enable attaching firewall engines to a `Gateway`
+* Enable `Gateway`-level firewall rule enforcement
+* Enable `HTTPRoute`-level firewall rule enforcement
+* Enable simple IP allow/deny lists
+* Provide documentation and best practices for implementations which describe
+  how firewall engines and rules can best be integrated into a Gateway API
+  implementation.
+
+## Non-Goals
+
+* Building a firewall implementation
+* Mesh-level support
+
+## API
+
+**TODO**: First PR will not include any implementation details, in favor of
+building consensus on the motivation, goals and non-goals first. _"How?"_ we
+implement shall be left open-ended until _"What?"_ and _"Why?"_ are solid.
+
+## Alternatives Considered
+
+### NetworkPolicy
+
+When discussing this originally the obvious question whether `NetworkPolicy`
+is sufficient, or should have some role in this, was asked. We do not consider
+it sufficient to resolve the goals unto itself. For the purposes of this GEP,
+we consider `NetworkPolicy` as an implementation detail at most: implementations
+_may_ choose how they enforce firewall rules, whether some of that is
+implemented with `NetworkPolicy` under the hood or not is up to them.
+
+## References
+
+* [GEP-1767: CORS](https://github.com/kubernetes-sigs/gateway-api/issues/1767)
+

geps/gep-3614/metadata.yaml

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+apiVersion: internal.gateway.networking.k8s.io/v1alpha1
+kind: GEPDetails
+number: 3614
+name: Gateway Firewall Support
+status: Provisional
+authors:
+  - shaneutt