Add mutable "viewers" list to DiscordRelayEvent

[NoahvdAa]

Information

This PR closes #5070.

Details

Proposed feature:

Expose a mutable list in the DiscordRelayEvent to change who receives the bridged message.

Environments tested:

OS: macOS

Java version: openjdk version "17.0.1" 2021-10-19

• Most recent Paper version (1.19.2 git-Paper-132)
• CraftBukkit/Spigot/Paper 1.12.2
• CraftBukkit 1.8.8

Demonstration:

@EventHandler
public void onDiscordRelay(DiscordRelayEvent event) {
    event.getViewers().removeIf(user -> user.getName().equals("Jroy"));
}

[JRoy]

approved for 2.20