feat(ux): namespace oncrete hook to check namespace exsistence

[prometherion]

Closes #868.

I tested the behaviour, and it's nice:

$: kubectl get tnt solar
NAME    STATE    NAMESPACE QUOTA   NAMESPACE COUNT   NODE SELECTOR   AGE
solar   Active   1                 1                                 27m

$: kubectl --as alice --as-group capsule.clastix.io create ns solar-test
namespace/solar-test created

$: kubectl --as alice --as-group capsule.clastix.io create ns solar-test
Error from server (AlreadyExists): namespaces "solar-test" already exists

$: kubectl --as alice --as-group capsule.clastix.io create ns solar-overquota
Error from server (Forbidden): admission webhook "namespaces.capsule.clastix.io" denied the request: Cannot exceed Namespace quota: please, reach out to the system administrators

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