Add support for other account systems

[ImplicitSaber]

Feature Description: It would be nice if you would add a config option to use an external account system. Maybe add an API for other plugins to use their own account system. Example:

public class AccountSystem implements DynmapAccSystem {

   @Override
   public DynmapAcc getAccByPlayer(UUID id) {
      //return the account
   }

   @Override
   public boolean regAccount(DynmapAcc account) {
      //return true if acc registered
   }

}

and then DynmapApi.registerAccountSystem(new AccountSystem());

• Additional context: I have a server with a self-developed plugin that manages accounts and would like a way to implement Dynmap.

[generrosity]

Mike would be happy if a contributor could submit some code, but I suspect in our Feature MegaList having SQL or Geyser or other custom logins have been requested before over the years but it is just really hard 😢

@FedUpWith-Tech is this one for tagging in the list?