docs: add initial egress gateway proposal

proposals/10-egress-gateways.md

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+# Egress Gateways
+
+* Authors: @shaneutt
+* Status: Proposed
+
+# What?
+
+Provide standards in Kubernetes to route traffic outside of the cluster.
+
+# Why?
+
+Applications are increasingly utilizing inference as a part of their logic.
+This may be for chatbots, knowledgebases, or a variety of other potential use
+cases. The inference workloads to support this may not always be on the same
+cluster as the requesting workload. Inference workloads may be on a separate
+cluster, as the organization centralizes them, or they may be located
+specifically for reasons of regionality. Even then, not all organizations are
+going to run inference workloads themselves, and will utilize 3rd party cloud
+services. All of this points to a need to provide standards for how Kubernetes
+workloads reach these external inference sources, and provide the same AI
+Gateway security, control and management capabilities that are required for the
+ingress use case.
+
+## User Stories
+
+* As a gateway admin I need to provide workloads within my cluster access to
+  services outside of my cluster, in particular cloud and otherwise hosted
+  services.
+
+* As a gateway admin I need to manage access tokens for 3rd party AI services
+  so that workloads on the cluster can perform inference within needing to
+  manage these secrets themselves, and so that I can manage access from all
+  workloads in a uniform manner.
+
+* As a gateway admin providing access and token management for 3rd party AI
+  cloud services to workloads, I need fail-over from one cloud provider to
+  others when the primary cloud provider is overwhelmed or in a failure state.
+
+* As a cluster admin I need to provide inference to workloads on my cluster,
+  but I provide a dedicated cluster for this so that I can manage it
+  separately.
+
+* As a cluster admin I need to provide inference to workloads on my cluster,
+  but I do not run AI workloads on Kubernetes. I use a cloud service to run
+  models (e.g. Vertex, Bedrock) and need workloads to have managed access to
+  that service to perform inference.
+
+* As a developer of an application that requires inference as part of its
+  function, I need my application to have access to external AI cloud services
+  which offer specific, specialized features only offered by that provider.
+
+* As a developer of an application that requires inference as part of its
+  function, I need fail-over to 3rd party providers if local AI workloads are
+  overwhelmed or in a failure state.
+
+* As a platform operator I need to attribute outbound traffic per namespace or
+  workload to enforce rate or API utilization limits.
+
+* As a compliance engineer I need to guarantee that outbound traffic to
+  third-party AI resources obeys regulatory restrictions such as region locks.
+
+## Goals
+
+* Define the standards for Gateways that route and manage traffic destined for
+  external resources outside of the cluster.
+* Define (or refine) the standards by which token management for Gateways can
+  be employed to enable access to backends that require auth.
+* Foundationally the standards for egress Gateways should be based on standards
+  based networking first, layering up to inference and agentic use cases.
+
+# How?
+
+TODO: in later PRs.
+
+
+> **This should be left blank until the "What?" and "Why?" are agreed upon,
+> as defining "How?" the goals are accomplished is not important unless we can
+> first even agree on what the problem is, and why we want to solve it.
+>
+> This section is fairly freeform, because (again) these proposals will
+> eventually find there way into any number of different final proposal formats
+> in other projects. However, the general guidance is to break things down into
+> highly focused sections as much as possible to help make things easier to
+> read and review. Long, unbroken walls of code and YAML in this document are
+> not advisable as that may increase the time it takes to review.
+
+# Additional Criteria
+
+The following are things we need to resolve before we can consider this
+proposal complete and ready to move out to other areas.
+
+- [ ] We need to decide how the multi-cluster aspect of egress gateways
+  interacts with the [GIE's multi-cluster proposal], if at all. This may end up
+  with multiple different multi-cluster options for users, so we'll need to be
+  clear about why there are multiple options, and what one solves over the
+  other. SIG MC needs to be a part of this conversation.
+- [ ] The Agentic Networking Subproject has a [proposal for external MCP/A2A]
+  services, making them a stakeholder for egress gateways as well. We need to
+  work with them to incorporate their user stories and requirements so that
+  what we ultimately ship covers the combined use cases.
+
+[GIE's multi-cluster proposal]:https://github.com/kubernetes-sigs/gateway-api-inference-extension/tree/main/docs/proposals/1374-multi-cluster-inference
+[proposal for external MCP/A2A]:https://docs.google.com/document/d/17kA-78gq25BgS2ElHMCd-zy__9clVL-GZQcHCm52854/edit?tab=t.0
+
+# Relevant Links
+
+* [Istio's implementation of Egress Gateways](https://istio.io/latest/docs/tasks/traffic-management/egress/egress-gateway/)