Remove metrics-server from uds-core, add as a documented prerequisite

[mjnagel]

Today we include metrics-server as an optional component in UDS Core. Metrics-server is required to allow for autoscaling (HPA) as well as enabling the kubectl top commands. Historically Big Bang added metrics-server to provide it for distributions of k8s that did not provide it out of the box (Konvoy being the primary one at the time).

In further evaluation of its usage, the inclusion of metrics-server has actually caused more pain than value. All known users of uds-core are on distros that provide metrics-server by default out of the box or as a standard addon:

• RKE2: default
• k3s/k3d: default
• EKS-D: default
• AKS: default
• EKS: standard addon

Based on it being present in most distros already the benefits we are getting from deploying it with core are:

• Hardened Image (ironbank or chainguard): Also generally possible with other deployment methods
• Network Policies: In the case of metrics-server these are fairly open due to how it is used in cluster
• Zarf/Airgap Ready: Definitely a benefit, although the "airgap methods" in various k8s distros should cover this as well

To reduce the confusion around this component it would be valuable to:

• Reenable metrics-server in uds-k3d
• Remove it from core
• Document metrics-server as a core requirement
• (possibly) Create a separate uds-package-metrics-server to continue to provide this optional functionality, outside of core

[mjnagel]

Discussed with some users - they have recently switched to EKS-d which does not provide metrics-server so having this as a standalone package or in core would be extremely beneficial to their deployments. I think for now we place to publish the functional layer of metrics-server to continue to enable this, but we could evaluate moving it out of core in the future.

cc @jacobbmay