Add a way to directly communicate with Paper servers

Requested Feature

What I would like to suggest is a feature which is not only tied to Velocity, but also Paper.

In short, I think it would be great to be able to directly communicate with backend Paper servers from the Velocity proxy. This could be done in a way similar to plugin messages (maybe a bit more wrapped together, allowing sending of more complex data types?), where either proxy or paper server send out a message and the receiver may receive and handle it.

Why is this needed?

I think this is mainly useful in case of sending "general" information without the need to rely on a specific player's connection.

Alternative Solutions

Currently, the only way to send information from or to backend servers is using plugin messages. This system has a few flaws though:

1. It requires an active player connection, meaning you can't just send data at any time (like server startup).
2. If not intercepted properly, messages from the backend to the proxy can result in a player being able to view those plugin messages.

Additional Information

I have decided to open the issue on the Velocity repo instead of the Paper one, since I feel like this is more a concern to developers who work on proxy servers instead of developers purely working with Paper.

Start by reviewing the existing plugin-message approach described in the issue and the communication paths between Velocity and Paper servers. Define how messages should work without an active player connection, including how complex data is represented and handled on each side. Done means a concrete, agreed protocol and implementation scope exists for both proxy and backend servers.