Expose occupied lobby slot information via the game API

The client currently only knows the number of human players, so lobbies with AI players or closed slots can report misleading player counts.

Currently, the information exposed by the server does not appear to distinguish between:

* Human players
* AI players
* Closed slots

Could the server expose enough information for clients to calculate the effective occupied slot count?

This request originates from the client feature request:

https://github.com/FAForever/downlords-faf-client/issues/3152