Tablist-changes for 1.19 (#761)

This change helps ensure player signatures are propagated correctly.
Signatures should never be removed, so to compensate for legacy plugins and for the
proxy api function we have to enforce this.

api/src/main/java/com/velocitypowered/api/proxy/player/TabListEntry.java

@@ -158,6 +158,8 @@ public interface TabListEntry extends KeyIdentifiable {
 
     /**
      * Sets the {@link IdentifiedKey} of the {@link TabListEntry}.
+     * <p>This is only intended and only works for players currently <b>not</b> connected to this proxy.</p>
+     * <p>For any player currently connected to this proxy this will be filled automatically.</p>
      *
      * @param playerKey key to set
      * @return {@code this}, for chaining