🐛 Re-wire additional indexers into informers creation

[embik]

While trying to use controller-runtime, I ran into the issue that I couldn't do List calls when the logical cluster was set in my ctx. The error I got was

Index with name cluster does not exist

Now I'm not an expert with those kube internals, but I spent some significant time delving into this and I discovered the following: In #10, kcp specific indexers were added. These were wired up properly. It seems we might have lost that in #42 (or maybe before), where the cache.Options field Indexers still exists but it is unused and therefore does not reach the part of the code where informers are created.

This PR re-wires the additional indexers and it solved the problem with calling List, but I have honestly no idea if it's the best way to do it or if there is some way to pass it already that i missed. The indexers in addInformerToMap looked pretty hardcoded to me, but again, limited knowledge.

I hope someone has a good enough understanding of controller-runtime's cache package that they can review this properly.

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[embik]

Closing in favour of #46. Much cleaner fix.