Add (cross-cluster) search

[joaquimrocha]

We should add a search view that searches for objects across clusters. This search should have filters to specify e.g. kinds of objects, namespaces, clusters, etc.

Acceptance Criteria:

• [ ] The search should be accessible from any view within a cluster.
• [ ] It should allow to filter what the search refers to (object kinds, namespaces, etc.), and what cluster to search in (or all, when omitted).
• [ ] It should show the results with links to their respective detailed views.