feat: Add memcache_customprefix

[kesselb]

Close #58334

Summary

Local version of #58334 with squashed commits.

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• Tests (unit, integration, api and/or acceptance) are included
• Screenshots before/after for front-end changes
• Documentation (manuals or wiki) has been updated or is not required
• Backports requested where applicable (ex: critical bugfixes)
• Labels added where applicable (ex: bug/enhancement, 3. to review, feature component)
• Milestone added for target branch/version (ex: 32.x for stable32)

AI (if applicable)

• The content of this PR was partly or fully generated using AI

[Earl0fPudding]

Thank you very much for all of your effort regarding this topic!

[Earl0fPudding]

/backport to stable32

[Earl0fPudding]

/backport to stable33

[backportbot]

The backport to stable33 failed. Please do this backport manually.

# Switch to the target branch and update it
git checkout stable33
git pull origin stable33

# Create the new backport branch
git checkout -b backport/58705/stable33

# Cherry pick the change from the commit sha1 of the change against the default branch
# This might cause conflicts, resolve them
git cherry-pick 2a2f42e7

# Push the cherry pick commit to the remote repository and open a pull request
git push origin backport/58705/stable33

Error: Failed to push branch backport/58705/stable33: remote: Invalid username or token. Password authentication is not supported for Git operations.
fatal: Authentication failed for 'https://github.com/nextcloud/server.git/'

---

Learn more about backports at https://docs.nextcloud.com/server/stable/go.php?to=developer-backports.