Add PlayerConfigurationResourcePackEvent to apply resource packs during configuration

[WouterGritter]

Introduces PlayerConfigurationResourcePackEvent, allowing plugins to apply resource packs while a player is still in the configuration state, before they enter play.

When it fires

The event is fired on every (re)configuration of a player:

• On initial post-login configuration (event.isFirstJoin() == true) - e.g. for network-wide packs.
• On every server-switch reconfiguration (event.isFirstJoin() == false) - e.g. for per-server packs.

If a plugin sets a pack via event.setResourcePack(...), the player is held in configuration until every pack reaches a terminal status:

• Required packs hold indefinitely until the client acknowledges (in practice the backend will time-out the player after 15 seconds, we catch this and gracefully disconnect the player with velocity.error.resource-pack-configuration-timeout).
• Optional packs hold for up to a timeout, then advance to play as if declined.

While holding, the proxy sends keepalives to the client itself (backends don't send keepalives after finishing configuration on their end).

Example

private ResourcePackInfo resourcePack;

@Subscribe
public void onProxyInitialize(ProxyInitializeEvent event) {
  byte[] hash = HexFormat.of().parseHex("8de90b010c37565361d47ea4a8b7c04ec5f6cf42");
  resourcePack = proxy.createResourcePackBuilder("http://example.com/resourcepack.zip")
      .setHash(hash)
      .setId(UUID.nameUUIDFromBytes(hash))
      .setShouldForce(true)
      .build();
}

@Subscribe
public void onConfigureResourcePack(PlayerConfigurationResourcePackEvent event) {
  if (event.isFirstJoin()) {
    event.setResourcePack(resourcePack);
  }
}

[R00tB33rMan]

I’ll test this later but LGTM from my POV