implemented server <-> client request/response for current_player_id

@bayadi @juckerj @alauber I think the whole setting and getting of unique playerIDs was not yet properly implemented. I remember talking with Benjamin about two options of communicating these IDs between client and server (who is allowed to play). In this branch I started implementing the necessary requests and responses, but to be honest I am quite out of my depth here. Tomorrow I won't be working until midday the earliest. feel free to use this branch and review/complete/fix/discard to your liking. You will see that two tests are failing - exactly the ones handling "allowed_to_play". GOOD LUCK 🍀