cmd/k8s-operator/deploy/chart: support nameOverride and fullnameOverride

The Helm chart hardcoded the operator's Deployment, ServiceAccount,
Role, and RoleBinding names to "operator".

Adds a standard _helpers.tpl with name/fullname template functions and
use them for the operator's resources, including the oauth Secret and
the cluster-scoped ClusterRole/ClusterRoleBinding. When neither
nameOverride nor fullnameOverride is set, the fullname resolves to the
historical hardcoded names rather than the release name, so existing
installations upgrade with no resource renames; rendering with default
values is byte-identical to before.

Fixes tailscale/tailscale#18232

Signed-off-by: chaosinthecrd <tom@tmlabs.co.uk>