feat: making default datastore optional

[prometherion]

@bsctl this change is required to support the DataStore Scheduler enterprise addon we aim to release.

After this change, Kamaji can be started with no --datastore field, forcing a Datastore assignment from user or addons.

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[bsctl]

@prometherion what about to have default datastore disabled by default?

[prometherion]

That would be very bad: even tho we released the v1.0.0 as stable and we have safe defaults for beginners, I would avoid providing a stumped Kamaji with no Datastore.

I'm expecting to get an HCP architecture just with a single command, without the complexity of installing the etcd with a different command, etc.

[prometherion]

Just tested, when creating a TenantControlPlane with missing DataStore the following logs are expected.

2024-10-30T15:07:44Z    INFO    the Tenant Control Plane doesn't have a DataStore assigned, and Kamaji is running with no default DataStore fallback    {"controller": "tenantcontrolplane", "controllerGroup": "kamaji.clastix.io", "controllerKind": "TenantControlPlane", "TenantControlPlane": {"name":"k8s-130","namespace":"kamaji-system"}, "namespace": "kamaji-system", "name": "k8s-130", "reconcileID": "20318484-cbb6-49e4-b515-2d14d9a87a37"}

If you restart the Kamaji Operator with the --datastore value, all the Tenant Control Plane instances with no DataStore will be set up with that value.

If CI is green, happy to get this merged.