Allow Client-Side Mods to Prevent Joining Lobbies

[Xilophor]

Is your feature request related to a problem? Please describe.
Some client-side mods can allow potential cheating/trolling due to the general lack of "sanity" checks the vanilla game has, and those mods have no current way of preventing players from joining servers. The only way to do this is via the [LethalRequiredPlugin] in #40 - but this requires both the host and client to have said mod.

Describe the solution you'd like
A new attribute mods can use to prevent players from joining vanilla servers.

Describe alternatives you've considered

1. Preventing any modded clients from joining vanilla servers.
2. Client-side mods that allow the potential for cheating should just add it as a required plugin for both the host and the client.

Additional context
N/A