Add alternative ingress controller support ADR

Signed-off-by: Brad Davidson <brad.davidson@rancher.com>

docs/adrs/008-traefik-ingress.md

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+# Support for Alternative Ingress Controllers
+
+Date: 2024-05-21
+
+## Status
+
+Accepted
+
+## Context
+
+RKE2 currently supports only a single ingress controller, ingress-nginx.
+It has been requested RKE2 support alternative ingress controllers, similar to how RKE2 supports multiple CNIs. 
+
+## Decision
+
+* A new --ingress-controller flag will be added; the default will be only `ingress-nginx` to preserve current behavior.
+* All selected ingress controllers will be deployed to the cluster.
+* The first selected ingress controller will be set as the default, via the `ingressclass.kubernetes.io/is-default-class` annotation
+  on the IngressClass resource.
+* Any packaged ingress controllers not listed in the flag value will be disabled, similar to how inactive packaged CNIs are handled.
+* RKE2 will package Traefik's HelmChart as a supported ingress controller, deploying as a Daemonset + ClusterIP Service
+  for parity with the `ingress-nginx` default configuration due to RKE2's lack of a default LoadBalancer controller.
+* RKE2 will use mirrored upstream Traefik images; custom-rebuilt hardened-traefik images will not be provided or supported.
+
+## Consequences
+
+* We will add an additional packaged component and CLI flag for ingress controller selection.
+* We will need to track updates to Traefik and the Traefik chart.
+* QA will need additional resources to test the new ingress controllers.