Set a metadata value for vanished players

[nik2143]

Information

This PR closes #3121.

Details

Proposed feature:

This PR add to the player a vanished metadata that is used from other plugins to check if the player is vanished

Environments tested:

OS: Windows 11

Java version: 16.0.1

• Most recent Paper version (1.XX.Y, git-Paper-BUILD)
• CraftBukkit/Spigot/Paper 1.12.2
• CraftBukkit 1.8.8

Demonstration:

        for (MetadataValue meta : player.getMetadata("vanished")) {
            if (meta.asBoolean()) return true;
        }

[JRoy]

Thanks for for your contribution!