Allow configuring the admin URL path of the Pi-hole instance

Allow configuring the admin URL path of the Pi-hole instance: a task in MiMo-V2.6-RL-harbor-code: MiMo-V2.6-RL Code (Harbor) (Harbor dataset). I run my Pi-hole behind a reverse proxy and the admin UI isn't reachable at the default /admin/ path — I've moved it to a different path prefix (e.g…

The task

I run my Pi-hole behind a reverse proxy and the admin UI isn't reachable at the default `/admin/` path — I've moved it to a different path prefix (e.g. `/pihole/`) so it fits into the rest of my URL layout. The Pi-hole UI itself works fine at that path.

Part of FineEnvs/MiMo-V2.6-RL-harbor-code.