Conditional sendPacket if Mod is installed on Remote

[BlayTheNinth]

Currently, mods advertised as server-only are not actually server-only because they try to send packets to the client and some or all loaders will close the connection on unexpected packets.

Affected mods include:

• TrashSlot
• Unbreakables

I think there may be a way to check for the existence of certain channels on the remote? But if not, we already have a packet for the version check that could be expanded for this.

Rough ideas:

Sending optional packets to clients

• Expand use of ServerboundModListMessage to also be sent on Neo/Forge
• some way for the server to lookup whether a given networked Balm mod is installed on the client
• new trySendTo method for sending a packet to a client that only does so for clients with the given Balm mod installed

Sending optional packets to servers

• Some way to specify whether a given Balm mod on the server should announce itself to the client
• a ClientboundModListMessage that only holds the announced mods
• some way for the client to lookup whether a given announced Balm mod is installed on the server
• new trySendToServer method for sending a packet to a server only if it has announced support for this mod