Add Native Linux Support

[gamingdoom]

This PR allows BeamMP to be compiled natively for Linux for use with the native version of the game, while making no changes to the Windows version.

Currently, the Linux version of BeamMP:

• Only works with the Steam version of the game (no registry on linux)
• Doesn't support Discord RPC

This PR also:

• Adds a GitHub actions manifest to build the Linux version.
• Renames network.h to network.hpp (it imports string so it is cpp)
• Adds extra error handling when parsing responses from server (necessary for proper function on Linux rapidjson)
• Adds vdf file parser (needed to find game) from https://github.com/TinyTinni/ValveFileVDF

[lionkor]

Hi, thanks for the contribution. Currently, we're in the process of making significant changes to the launcher, so this will not be merged at this time.

Changes include a rework of the protocol.

[thevbw]

merge, coward

[lionkor]

I'd happily introduce some of these ideas to the new version, but as you saw from the code, the current launcher is not in an acceptable state. I've locked the conversation for now, @thevbw please direct your comments to /dev/null

[lionkor]

Will fix this up ourselves :) @gamingdoom you ok with licensing your code AGPL-3.0?