Feature: per-server info forwarding mode

[MarcoLvr]

What's this?
With this edit every server can use a different forwarding mode to obtain and forward player info. For instance, if you are running a 1.12 (or lower version) server on a velocity proxy with MODERN player info forwarding the server doesn't support MODERN forwarding. So you can set LEGACY forwarding mode for that server and velocity will use ONLY FOR THAT SERVER the legacy forwarding mode. The default mode is "INHERIT" which uses the player info forwarding mode set in the config.

Note: the INHERIT mode doesn't physically exists. Just set the server forwarding mode to NULL or avoid valuing it in the config

The main configuration now supports two syntaxes for describing a server:

classic syntax (INHERIT mode): server = "address:port"
new syntax: server = {address = "address:port", forwarding-mode = "MODE"}

all servers created without defining a forwarding mode will use the INHERIT mode.

Example of use
Imagine a simple network with a lobby, a 1.8 game mode (like bedwars) and maybe a Fabric 1.21 server.
Fabric needs MODERN forwarding but the 1.8 server cannot use it. So you have two choices:

1. Set the main forwarding mode to legacy (allow <1.13 players to join) and set MODERN mode only for the fabric server.

[...]
player-info-forwarding-mode = "legacy"
[...]
[servers]
try = ["lobby"]
lobby = "localhost:25566" //we leave this in INHERIT mode because in this case the lobby could be a 1.8 server too
bedwars = "localhost:25567" //we leave this in INHERIT mode because the main mode is legacy
modded =  { address = "localhost:25568", forwarding-mode = "MODERN" } //fabric needs MODERN forwarding
[...]

2. Set the main forwarding mode to MODERN and set LEGACY mode only for the 1.8 server.

[...]
player-info-forwarding-mode = "modern"
[...]
[servers]
try = ["lobby"]
lobby = "localhost:25566" //we leave this in INHERIT mode because in this case the lobby could be a >1.13 server
bedwars = { address = "localhost:25567", forwarding-mode = "LEGACY" }  //bedwars (1.8 server) needs legacy mode
modded =  "localhost:25568" //we leave this in INHERIT mode because the main mode is modern
[...]

This system could be very useful for these types of situations!

[MarcoLvr]

did everything! Let me know if there's any other change to do

[4drian3d]

https://streamable.com/axixhc

lgtm

[MarcoLvr]

@Emilxyz we are waiting for you :)

[Emilxyz]

we are waiting for you

I'm not a team member, just sharing my thoughts, so my approval is not actually required. Also a friendly reminder that everyone is doing this on their free time, so it's not really needed to bump a review request after only a day.

I gave this another look and it basically lgtm other than these minor things in review comments.

Another thought I had (but I would wait with doing any changes) is that maybe we can just use ServerInfo for this instead of adding a new type, they basically share the same fields and probably will continue to do so in the future as well.

Also there is the issue of the planned config api switch which is holding up other things already that would make changes to the config, so be prepared for this pr to also be affected by this