feat: add feature toggle to allow controller default tags to be overridden by annotation supplied tags

[mwhittington21]

Issue

#4030

Description

Adds an option to override the existing behaviour where tags added to the controller via the --default-tags argument will override any annotation-specified tags. When the feature is enabled, annotation-specified tags will now override default tags. This had to be added as a toggle-able feature because there are likely many customers that are relying on the current behaviour.

The flag for the new behaviour has been added as a feature gate. It could also be an argument, but the main reason I chose to use a feature gate was to minimise refactoring involved. Feature gates are already passed through the entire codebase, so the amount of scaffolding changes were minimised. Tests were easier to write, and existing tests needed minimal changes.

The feature gate is called EnableDefaultTagsLowPriority, indicating that when enabled it will change the default tags to be of lower priority than other tags.

The majority of changes are in tests. Every other change is just an extra conditional statement in all the areas of code that merge two dicts of tags. I chose to add these conditional statements rather than change the tag merge logic and add an argument as it would only touch the parts of the code related to tags. Anything else using that function remains untouched.

Checklist

• Added tests that cover your change (if possible)
• Added/modified documentation as required (such as the README.md, or the docs directory)
• Manually tested
• Made sure the title of the PR is a good description that can go into the release notes

BONUS POINTS checklist: complete for good vibes and maybe prizes?! 🤯

• Backfilled missing tests for code in same general area 🎉
• Refactored something and made the world a better place 🌟

[k8s-ci-robot]

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/ok-to-test