[core] Refactor "cluster" and "clusters" package

[ricoberger]

This commit refactors the "cluster" and "clusters" package. We
introduced some new interfaces and now creating "clients" in both
packages which must implement the interfaces to interact with a
Kubernetes cluster.

This was required to increase the test coverage in the future, by
mocking the clients to interact with all / one Kubernetes cluser.

With this change with had to adjust multiple packages and plugins where
the clusters client was used. For all plugins which are not interacting
with a Kubernetes cluster we removed the clusters client as parameter.

• I added a CHANGELOG entry for this change.
• I adjusted the corresponding documentation for this change.
• I adjusted the values.yaml file and the documentation for all values.

[codecov-commenter]

Codecov Report

Merging #263 (8ef3cd3) into main (a9d15fb) will increase coverage by 6.20%.
The diff coverage is 52.53%.

@@            Coverage Diff             @@
##             main     #263      +/-   ##
==========================================
+ Coverage   16.20%   22.41%   +6.20%     
==========================================
  Files          89       89              
  Lines        7533     7539       +6     
==========================================
+ Hits         1221     1690     +469     
+ Misses       6295     5818     -477     
- Partials       17       31      +14     

Impacted Files	Coverage Δ
cmd/kobs/kobs.go	8.10% <0.00%> (ø)
pkg/api/api.go	5.26% <0.00%> (ø)
pkg/api/clusters/cluster/cluster.go	0.00% <0.00%> (ø)
pkg/api/clusters/router.go	0.00% <0.00%> (ø)
pkg/api/middleware/auth/auth.go	0.00% <0.00%> (ø)
pkg/api/middleware/auth/handler.go	29.41% <0.00%> (ø)
plugins/applications/pkg/teams/teams.go	0.00% <0.00%> (ø)
plugins/applications/pkg/topology/topology.go	14.51% <0.00%> (ø)
plugins/flux/pkg/sync/sync.go	0.00% <0.00%> (ø)
plugins/resources/resources.go	6.10% <20.00%> (+6.10%)	⬆️
... and 42 more

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