cmd/k8s-operator: generate static kube manifests from the Helm chart. (#10436)

* cmd/k8s-operator: generate static manifests from Helm charts

This is done to ensure that there is a single source of truth
for the operator kube manifests.
Also adds linux node selector to the static manifests as
this was added as a default to the Helm chart.

Static manifests can now be generated by running
`go generate tailscale.com/cmd/k8s-operator`.

Updates tailscale/tailscale#9222

Signed-off-by: Irbe Krumina <irbe@tailscale.com>

cmd/k8s-operator/deploy/manifests/templates/03-secret.yaml

@@ -0,0 +1,9 @@
+apiVersion: v1
+kind: Secret
+metadata:
+  name: operator-oauth
+  namespace: tailscale
+stringData:
+  client_id: # SET CLIENT ID HERE
+  client_secret: # SET CLIENT SECRET HERE
+---