Add configurable labels to namespaces

[eberlep]

Right now, we create a new namespace for each database, And our log-shipping uses labes of the namespace to forward and route logs.

We already add a postgres.database.fits.cloud/managed-by label to new namespaces, but should allow custom labels as well.

The question is: per database (in the custom ressource) or per postgreslet (in it's configmap)?

I think a would prefer a mixture of both, so that e.g. log-shipping in general could be enabled per postgreslet installation, but the destination of the logs could be configured per database.

And I think we already have a labels feature for databases, we would just have to use them for namespaces as well.