feat: join states for external servers

[FllipEis]

Adds external-server-joinstates so servers that aren't managed by SimpleCloud can have their own join state. Builds on #25 by @TheoNX.

external-server-joinstates:
  queue-limbo: public

• Cloud servers resolve like before (service → group → persistent server → initial-state)
• The map is only used when the server isn't known to the cloud, matched case-insensitively, otherwise initial-state
• Unknown join states in the map are logged on load
• The migrator adds the key to existing configs

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