SteamMatchmaking - creating, retrieving, listing, joining and leaving lobbies.

[LossyDragon]

Description

From PR 704 on SteamKit

Add SteamMatchmaking - creating, retrieving, listing, joining and leaving lobbies.

Note: I only made up a quick unit test for FriendCache.java. Everything else has no unit testing and should be considered unstable until proven.

I havent done physical testing myself, and this is a backport from Java 11.

Closes #70

Checklist

• [x] Code compiles correctly
• [x] All tests passing
• [x] Samples run successfully
• [x] Extended the README / documentation, if necessary