Add SteamWorks integration

[Fohlen]

We want to have SteamWorks integration in the game. Have a look at the SDK documentation.

In particular we want the following features:

• indicate in Steam that you are playing the game
• show to your friends in which server you are playing
  • when clicking the "Show server" button it should show the server and who is currently playing (which mode, time, name)
• make it possible to join the same server as your friends

[Croydon]

Will the Steam integration stay optional or will Tomatenquark be distributed as Steam-only?

[Fohlen]

The Steam integration should always be optional. I imagine something like a ENABLE_STEAM compilation flag that will enable/disable the integration (similar to Red Eclipse)

[Fohlen]

Okay, a first step to take is:

• include Steam SDK according to their guide
• add a AUTH network package for Steam authentication
• add a server variable so the server can decide between
  • disabled mode (no Steam verification is performed)
  • optional mode (clients can and can not authenticate via Steam)
  • locked mode (clients must authenticate via Steam otherwise they cannot play)

This helps massively fight cheaters because of the VAC. Further some remarks:

• add a compilation flag (ENABLE_STEAM) to all platforms
• never commit or compile the SteamWorks SDK into Github/Github Releases because it is proprietary
• additionally offer the server for download via the SteamWorks SDK ('App Tool with custom AppID')

This seems straightforward to be done and I will start working on it after the release fragfest.

[Fohlen]

We can use the following functionality https://framagit.org/khanat/khanat/blob/93f5836fd5372c0b07d0be569f9c8e285239d899/code/CMakeModules/FindSteam.cmake to work with Steam. This means that a implementation can be agnostic (just drop the SDK folder anywhere and it will automagically add ENABLE_STEAM as a compilation definition).