Make finalizer's name configurable

[nolar]

What do these changes do?

Make finalizer's name configurable, thus allowing multiple operator with different identities to handle the same resources with advanced handlers (anything different than on.event).

Description

As a side-effect of #331, the operator gained possibility to change its own identity by using different prefixes or fields instead of the hard-coded Kopf's names/domains. This allowed multiple operators handling the same resources with advanced handlers (anything different than on.event; e.g. on.create, on.update, on.delete, etc; also, kopf.daemon & kopf.timer).

However, only one finalizer was used for all operators: kopf.zalando.org/KopfFinalizerMarker (hard-coded). This could lead to different operators possibly colliding with each other.

With this PR, it is now possible to also configure the finalizer's name/domain. This is the final thing with operator's identity left that could not be configured.

Issues/PRs

Issues: #23 #283

Type of changes

• New feature (non-breaking change which adds functionality)

Checklist

• The code addresses only the mentioned problem, and this problem only
• I think the code is well written
• Unit tests for the changes exist
• Documentation reflects the changes
• If you provide code modification, please add yourself to CONTRIBUTORS.txt

[zincr]

🤖 zincr found 0 problems , 0 warnings

✅ Large Commits
✅ Approvals
✅ Specification
✅ Dependency Licensing

[zincr]

🤖 zincr found 1 problem , 0 warnings

❌ Approvals
✅ Large Commits
✅ Specification
✅ Dependency Licensing

Details on how to resolve are provided below

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Approvals

All proposed changes must be reviewed by project maintainers before they can be merged

Not enough people have approved this pull request - please ensure that 1 additional user, who have not contributed to this pull request approve the changes.

• ✅ Approved by PR author @nolar
• ❌ 1 additional approval needed