cmd/k8s-operator: move proxyclass reconciler into its own package (#19695)

This commit moves the proxyclass reconciliation logic from its original
home in the rather large main package of the k8s-operator to a dedicated
package within `k8s-operator/reconciler/proxyclass`.

Closes: https://github.com/tailscale/corp/issues/37085

Signed-off-by: David Bond <davidsbond93@gmail.com>