Investigate on the possibility to disable the self provisioning plugin on some namespaces

[lavocatt]

We've received valuable feedback from @mkrutov:

So suppose you are a customer and you're running a broker on your cluster, so you receive an update with new shiny feature you would want to check out. Even if its only dev preview, it sounds interesting, right?
So you would create a new namespace, test or something, in which you would play around with the SPP. But SPP would be available in all namespaces, so if operator does anything different when SPP is present it would affect your previous deployment as well.
I know you're supposed to use a test cluster for this kind of thing, but than not everyone would, so

We need to investigate on the ability to deactivate the self provisioning plugin on demand. An early idea is to rely on feature flags to do so. We haven't work with those yet, so let's figure out how they work, what are their limitations and then let's come up with a plan to limit the SPP to some namespaces.