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Fix potential unexpected object mutation that can lead to data races
**What this PR does / why we need it**:
In #51526 I introduced an optimization - do a deep copy instead of to and from JSON roundtrip to convert anything that implements `runtime.Unstructured`. I just discovered that the method that is used there `UnstructuredContent()` in both `Unstructured` and `UnstructuredList` may mutate the original object.
2008750398/staging/src/k8s.io/apimachinery/pkg/apis/meta/v1/unstructured/unstructured.go (L87-L92)7c10cbc642/staging/src/k8s.io/apimachinery/pkg/apis/meta/v1/unstructured/unstructured_list.go (L58-L75)
This is problematic because previously (before #51526) there was no mutation and because this is unexpected and may lead to data races - it is bad behaviour to mutate original object when you just want a copy of it.
This PR fixes the issue.
Without the fix the tests I've added are failing because when comparison is done original object is not the same:
```
converter_test.go:154: Object changed, diff:
object.Object[items]:
a: []interface {}{}
b: <nil>
converter_test.go:154: Object changed, diff:
object.Object[items]:
a: []interface {}{map[string]interface {}{"kind":"Pod"}}
b: <nil>
```
However the underlying issue is not fixed here - `UnstructuredContent()` is brittle and dangerous. Method name does not imply that it mutates data when you call it. And godoc does not mention that either:
509df603b1/staging/src/k8s.io/apimachinery/pkg/runtime/interfaces.go (L233-L249)
Something needs to be done about it IMO.
Also `UnstructuredContent()` implementation in `UnstructuredList` does not implement the behaviour required by godoc in `runtime.Unstructured`.
**Release note**:
```release-note
NONE
```
/kind bug
/sig api-machinery
/assign @sttts
2cbb07a439