Add Permission for XP Multiplier

[JericNisperos]

Is your feature request related to a problem? Please describe.

• Not really a problem but a feature where players can obtain permanent or temporary multipliers using Permission. Since permission is very flexible, you can add permanent or add it temporarily. So adding a new feature where we can specify the multiplier and the permission will be helpful.

Describe the solution you'd like

• If it's fine, Adding a new argument from config or just a new file with an option to specify the multiplier.
  Ex.
  Multiplier: 2.0
  Permission: ecopets.multiplier.double

Describe alternatives you've considered

• You can create a default permission instead with the following multipliers: 1.25, 1.5, 1.75, 2.0

Additional context

• To avoid bugs, it can only respect the highest multiplier that was added to a player.