Add support for BattleMetrics reserved slot system

[FlorianSW]

BattleMetrics provides a system to have reserved slots on a server. This has a similar integration like CFTools, using a HTTP based API.

Things that should be included:

• Configurable perk for reserved slots (days/permanent)
• Support for multiple servers and organizations
• The perk should be visible as "owned" on the start page for the donator

[FlorianSW]

There's currently an issue with the BM API (or a misuse from my side, idk), which does not allow me to fetch the reserved slots for a player ID for sure (it returns the slots, which were deleted). I'm trying to clarify that (https://discord.com/channels/202199157224636417/245335846088081408/887799519385878539) right now.