🐛 RuntimeSDK: some bugfixes

[sbueringer]

What this PR does / why we need it:
This PR fixes some bugs which have been introduced by recent PRs:

1. The ExtensionConfig controller need permissions to list namespaces
2. The ExtensionConfig controller requires the client to be set in main.go otherwise we're getting a panic when we try to List namespaces
3. We have to select the namespace via expressions as / is not allowed in the label selector

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
Fixes #

[killianmuldoon]

/lgtm

[fabriziopandini]

lgtm pending comment above.
to use the const we need #6632 to merge first, which makes sense for me

[sbueringer]

lgtm pending comment above. to use the const we need #6632 to merge first, which makes sense for me

Let's move the fix to #6632

[sbueringer]

This PR should now be unblocked

[chrischdi]

/lgtm

[vincepri]

/approve

[k8s-ci-robot]

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