Allow adlist duplicates

[DL6ER]

What does this implement/fix?

Allow narrowing down the (ad)list type using the (optional) query parameter ?type={allow,block}.

Accompanying change related to https://github.com/pi-hole/pi-hole/pull/5572. See reasoning therein.

This is a feature request submitted on Discourse (see link below).

[!NOTE] This pull requests requires all three Pi-hole components to be checked out

pihole checkout core tweak/allow_adlist_dups
pihole checkout web tweak/allow_adlist_dups
pihole checkout ftl tweak/allow_adlist_dups

for testing

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Related issue or feature (if applicable): N/A

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

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By submitting this pull request, I confirm the following:

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/lists-cant-add-identical-url-twice-blocklist-allowlist/68261/12

[github-actions[bot]]

This pull request has conflicts, please resolve those before we can evaluate the pull request.

[github-actions[bot]]

Conflicts have been resolved.

[DL6ER]

Has been confirmed working on the related Discourse thread

[DL6ER]

Merge only once https://github.com/pi-hole/pi-hole/pull/5572 and https://github.com/pi-hole/web/pull/2958 are approved as well as these branches depend on each other