refact: use Set structs instead of map[]struct{}

[vflaux]

What does it do ?

Use k8s.io/utils/set or k8s.io/apimachinery/pkg/util/sets.Set or instead of map[]struct{}.

k8s.io/utils/set is restricted in accepted types while k8s.io/apimachinery/pkg/util/sets.Set is not.
k8s.io/utils/set provide a SortedList() method which is useful in some cases with String sets.

Motivation

While map[]struct{} is idiomatic to implement a set, it's clearer and more readable to use a dedicated Set struct.

More

• Yes, this PR title follows Conventional Commits
• Yes, I added unit tests
• Yes, I updated end user documentation accordingly

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[k8s-ci-robot]

@vflaux: The following test failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
pull-external-dns-unit-test	af1e25f	link	true	/test pull-external-dns-unit-test

Full PR test history. Your PR dashboard. Please help us cut down on flakes by linking to an open issue when you hit one in your PR.

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