[networking] Improve connection events

[SpaceWalkerRS]

The LOGIN, PLAY_READY, and DISCONNECT events currently provide very little context. The client-side events give a Minecraft instance, and the server-side events give a MinecraftServer instance and a ServerPlayerEntity instance.

This PR replaces those with a single context object through which these instances and other relevant data can be accessed.

The PR also fixes the DISCONNECT event not firing if initiated by the remote side. For example, the client-side DISCONNECT event would not fire if the player was kicked from the server or had a connection issue. And the server-side DISCONNECT event would not fire if the player left the game through the game menu or by closing the game. These issues have been fixed.

Here are some example usages of the new API:

ClientConnectionEvents.LOGIN.register(context -> {
	Minecraft minecraft = context.minecraft();

	if (context.isServerLocal()) {
		String worldName = context.worldName();
	} else {
		String serverAddress = context.serverAddress();
		int serverPort = context.serverPort();
	}
});
ClientConnectionEvents.DISCONNECT.register(context -> {
	TextComponent disconnectReason = context.disconnectReason();
});

ServerConnectionEvents.LOGIN.register(context -> {
	MinecraftServer server = context.server();
	ServerPlayerEntity player = context.player();
});
ServerConnectionEvents.DISCONNECT.register(context -> {
	TextComponent disconnectReason = context.disconnectReason();
});