chore(source): move cache informer to dedicated folder

[ivankatliarchuk]

What does it do ?

Motivation

I was looking at issues, example

• Unnecessary access to v1.Pod lead to timeout. #4955
• Too many Kubernetes resources will cause "failed to sync *v1.Pod: context deadline exceeded" #3173
• failed to sync *v1.Node (but I didn't request --source=node) #3169

Most of them related timeouts. This PR is not a fix for any of them. There is a flag https://github.com/kubernetes-sigs/external-dns/blob/36bc7d6bc47d4d8862aa02f88ac57d0608919a2a/docs/flags.md?plain=1#L12C1-L12C2 --request-timeout=, but the config is not respected, the timeout is static.

In this PR I've

• added more unit tests
• moved cache to dedicated folder
• different source do use different aliases for imports, fixed this
• added more units tests

This will simplify in follow-up to

• support sync request timeout in cache sync with the flag
• will simplify in fiture to move sources to specific folders
• create an interface, to simplify unit testing

More

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

[mloiseleur]

/lgtm
/approve

[k8s-ci-robot]

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