Add option to ignore unknown DHCP clients

[DL6ER]

What does this implement/fix?

Add new dhcp.ignoreUnknownClients option which has been requested on Discourse. It works as follows:

This PR also features a second commit updating forgotten changes into test/pihole.toml and adds a CI test to prevent forgetting to update this file in the future.

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Related issue or feature (if applicable): https://discourse.pi-hole.net/t/add-checkbox-to-block-dhcp-for-unknown-hosts/70485

Pull request in docs with documentation (if applicable): N/A

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1. I have read and understood the contributors guide, as well as this entire template. I understand which branch to base my commits and Pull Requests against.
2. I have commented my proposed changes within the code.
3. I am willing to help maintain this change if there are issues with it later.
4. It is compatible with the EUPL 1.2 license
5. I have squashed any insignificant commits. (git rebase)

Checklist:

• [x] The code change is tested and works locally.
• [x] I based my code and PRs against the repositories developmental branch.
• [x] I signed off all commits. Pi-hole enforces the DCO for all contributions
• [x] I signed all my commits. Pi-hole requires signatures to verify authorship
• [x] I have read the above and my PR is ready for review.

[pralor-bot]

This pull request has been mentioned on Pi-hole Userspace. There might be relevant details there:

https://discourse.pi-hole.net/t/add-checkbox-to-block-dhcp-for-unknown-hosts/70485/5

[rdwebdesign]

Should we add this option on the web interface, Settings > DHCP page?

Something like this?

Or this should be only in "All settings" page?

[DL6ER]

@rdwebdesign In the end, yes. But it'll be a separate PR and I haven't had the chance to do it myself, so far. Please go ahead :-)