Add libraries for game protobufs

[Longi94]

Use protobufs from this repo https://github.com/SteamDatabase/Protobufs

[LossyDragon]

Kinda merging #39 into this.

Would be nice to make the link a sub module. Then have some automation to convert those proto files into the format JavaSteam uses.

Then game specific protos can be added as library additions too.

Just a thought for a rainy day 😄