[FEATURE REQUEST] Add command for offline-to-premium account migration

[Nektoo]

It would be useful to add a command similar to LimboAuth's /premium command.

Example scenario:

1. The player registers an account:
   /register password

2. Later, the player buys Minecraft or wants to use their existing Microsoft/Mojang account.

3. The player executes:
   /premium

4. VeloAuth verifies whether the Minecraft account is a premium account and switches the account to premium authentication.

After the migration:

• The player no longer needs to enter a password.
• Authentication is handled through the Mojang/Microsoft session.
• The account keeps its existing data (permissions, statistics, inventory, etc.).

This would make the transition from offline authentication to premium authentication more convenient for servers that support both cracked and premium players