refactor: unify the ObjectRef and GroupingKey types

[SOF3]

Description

Related issues

Special notes for your reviewer:

[codecov-commenter]

Codecov Report

Patch coverage: 6.45% and project coverage change: -0.81% ⚠️

Comparison is base (f511c44) 37.63% compared to head (880213c) 36.82%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #158      +/-   ##
==========================================
- Coverage   37.63%   36.82%   -0.81%     
==========================================
  Files          32       33       +1     
  Lines        2559     2626      +67     
==========================================
+ Hits          963      967       +4     
- Misses       1533     1596      +63     
  Partials       63       63              

Files Changed	Coverage Δ
pkg/diff/cache/interface.go	11.26% <0.00%> (ø)
pkg/diff/cache/memory_wrapper.go	0.00% <0.00%> (ø)
pkg/util/object/rich.go	0.00% <0.00%> (ø)
pkg/util/object/key.go	5.00% <5.00%> (ø)
pkg/k8s/objectcache/objectcache.go	51.35% <80.00%> (ø)

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