[WIP] Refactor tags service

[Jont828]

What type of PR is this?

What this PR does / why we need it: Although the tags service doesn't quite fit into the async and ResourceSpecGetter interfaces, we can still do some refactoring to clean up the code.

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
Fixes #2142

Special notes for your reviewer:

Please confirm that if this PR changes any image versions, then that's the sole change this PR makes.

TODOs:

• squashed commits
• includes documentation
• adds unit tests

Release note:

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[CecileRobertMichon]

comment needs to be updated

[CecileRobertMichon]

@Jont828 I think this smaller refactor makes sense. Please comment once it's ready for review, right now it's still failing tests and is marked as WIP.

[Jont828]

Sounds good, I was holding off on the tests until we decide to go with this refactor approach. I'll go ahead and add tests soon.

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/lifecycle stale

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/lifecycle rotten

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@Jont828: The following tests 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-cluster-api-provider-azure-coverage	50097cc	link	false	/test pull-cluster-api-provider-azure-coverage
pull-cluster-api-provider-azure-verify	be76d1b	link	true	/test pull-cluster-api-provider-azure-verify
pull-cluster-api-provider-azure-test	be76d1b	link	true	/test pull-cluster-api-provider-azure-test
pull-cluster-api-provider-azure-ci-entrypoint	be76d1b	link	true	/test pull-cluster-api-provider-azure-ci-entrypoint
pull-cluster-api-provider-azure-e2e-exp	be76d1b	link	false	/test pull-cluster-api-provider-azure-e2e-exp
pull-cluster-api-provider-azure-e2e	be76d1b	link	true	/test pull-cluster-api-provider-azure-e2e

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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[Jont828]

After discussing with @CecileRobertMichon and @jackfrancis I think we can close out this refactor since the tagsChanged() function is quite difficult to understand or change in a way that ensures it actually works.