Thread DefaultResourceModifierConfigMap from server config through to
restoreReconciler. Refactor validateAndComplete to use a shared
loadResourceModifierConfigMap helper that handles both default and
per-restore ConfigMap loading.
Precedence: per-restore modifier takes exclusive precedence over the
default. Default ConfigMap errors are non-fatal (warn and proceed).
SkipDefaultResourceModifier opt-out is respected.
Includes unit tests covering: default-only, per-restore override,
skip flag, missing default (non-fatal), missing per-restore (fatal),
and no modifier configured.
Signed-off-by: Shubham Pampattiwar <spampatt@redhat.com>