fix(operator): add spec.services.onlineStore.disabled to opt out of the online store

The operator always deploys an online store, forcing a serving pod and
persistence onto registry-only or offline-only FeatureStores (#6586). Add a
`disabled` field on the online store spec to opt those deployments out
explicitly. Deployment is gated through isOnlineStore(), so disabling also
skips the serving pod, metrics, and service monitor. The default is unchanged
-- omitting the online store block still deploys it with defaults -- so this
is not a breaking change on upgrade.

Signed-off-by: Nikolaus Schuetz <nikolauspschuetz@gmail.com>