Add method to get all linked players in DiscordLinkService

[carlodrift]

Feature description

Add a method getAllLinkedPlayers to the DiscordLinkService interface that returns a map of all linked players, where the key is the Discord ID and the value is the Minecraft UUID.

How the feature is useful

This feature will allow server owners and developers to easily retrieve all linked players, facilitating better integration and management of Discord and Minecraft accounts.