design: Run Envoy Gateway locally on host

Relates to envoyproxy#1001
& envoyproxy#1035

Signed-off-by: Arko Dasgupta <arko@tetrate.io>

docs/latest/design/local-envoy-gateway.md

@@ -0,0 +1,48 @@
+# Running Envoy Gateway locally
+
+## Overview
+
+Today, Envoy Gateway runs only on Kubernetes. This is an ideal solution
+when the applications are running in Kubernetes.
+However there might be cases when the applications are running on the host which would
+require Envoy Gateway to run locally.
+
+## Goals
+
+* Define an API to allow Envoy Gateway to retrieve configuration while running locally.
+* Define an API to allow Envoy Gateway to deploy the managed Envoy Proxy fleet on the host
+machine.
+
+## Non Goals
+
+* Support multiple ways to retrieve configuration while running locally.
+* Support multple ways to deploy the Envoy Proxy fleet locally on the host.
+
+## API
+
+* The `provider` field within the `EnvoyGateway` configuration only supports
+`Kubernetes` today which provides two features - the ability to retrieve
+resources from the Kubernetes API Server as well as deploy the managed
+Envoy Proxy fleet on Kubernetes. 
+* This document proposes adding a new top level `provider` type called `Custom` 
+with two fields called `resource` and `infrastructure` to allow the user to configure
+the sub providers for providing resource configuration and an infrastructure to deploy
+the Envoy Proxy data plane in.
+* A `File` resource provider will be introduced to enable retrieveing configuration locally
+by reading from the configuration from a file.
+* A `Host` infrastructure provider will be introduced to allow Envoy Gateway to spawn a 
+Envoy Proxy child process on the host.
+
+Here is an example configuration
+```
+provider:
+  type: Custom
+  custom:
+    resource:
+      type: File
+      file:
+        path: "config.yaml"
+    infrastructure:
+      type: Host
+      host: {}
+```

docs/latest/design_docs.rst

@@ -17,3 +17,4 @@ Learn about the internal details of Envoy Gateway.
   design/request-authentication
   design/bootstrap
   design/extending-envoy-gateway
+  design/local-envoy-gateway