Add custom RetroAchievements host support to standalone Dolphin

[misantronic]

Summary

• Standalone Dolphin's `cheevos_dolphin.sh` fully regenerates `RetroAchievements.ini` on every launch, with no way to point achievements traffic at a custom host.
• RetroArch already supports this via `global.retroarch.cheevos_custom_host` (see `batocera.conf`). This adds the equivalent for Dolphin: a new, emulator-agnostic `global.retroachievements.customhost` setting, written into Dolphin's `RetroAchievements.ini` as `HostUrl`.
• When unset, `get_setting` returns an empty string, so `HostUrl` stays empty and behavior is unchanged from today (Dolphin falls back to the official RetroAchievements server).

Motivation

Building ROCKNIX support for RAOfflineProxy. RetroArch and standalone PPSSPP already support the custom-host pattern; Dolphin is the remaining gap since its cheevos script does a full config overwrite instead of a targeted `sed` edit like PPSSPP's does.