Allow for custom property comparators

[sheerun]

It build on top of #1534 and fixes #1463 with the same base idea but (allow providers to define their property comparator) but also enhancements from #1534 which this PR succeeds. The main changes are:

1. Added tests to plan_test.go that verify logic in general case
2. Added tests to cloudflare_test.go that actually test this issue is fixed
3. Renamed "Attribute" to "Property" to be consistent with "ProviderSpecificProperty"
4. Comparator is passed as one of Plan attributes instead of being passed to Calculate
5. Signature of property comparator changed from func (string, *string, *string) bool to func (string, string, string) bool to avoid issues with improper handling of nil values (with current signature empty string is treated the same way as nil value, if there is a need to treat them differently in the future, the signature can be of course changed back).
6. Simplified shouldUpdateProviderSpecific code and made it O(n)

[sheerun]

/assign @njuettner

[sheerun]

I've checked this change in live application and it seems to be working

[sheerun]

It's rebased and ready to be reviewed

[sheerun]

rebased

[sheerun]

/assign @Raffo

[sheerun]

@tariq1890 @ytsarev Maybe you want to review this one as well? It allows any provider to define custom comparators which can help with custom properties or bugs that cause repeated updates

[Raffo]

Thanks for the PR @sheerun , I think that's great! I added a few comments, but I think overall this is good to ship.
Did you test an image with that? We are aiming at a release of externalDNS next week and I want to make sure that this would be good to ship in case we get it in.

[sheerun]

@Raffo I fixed your suggestions, mind to approve? :)

[sheerun]

/lgtm

[k8s-ci-robot]

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/lgtm

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[Raffo]

Sorry for nit-picking, here's a few more suggestions.

[sheerun]

No problem, updated

[Raffo]

Restarting the tests.

[Raffo]

The failure seems to be a legit one @sheerun :

plan/plan.go:24:2: imported and not used: "github.com/sirupsen/logrus" as log

can you please fix it?

[Raffo]

/approve
/lgtm

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: Raffo, sheerun

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