Implement Elo rating

[marein]

Problem description

1. Players want to know how they perform over time.
2. Players want to pair with equally strong players.
3. Players want to know where they are on a scoreboard.

Possible solution
Create a new class named Player inside the context ConnectFour. Below there's an example structure of this class. The username is there as an example of how this issue supplements issue #14.

final class Player
{
    public function __construct(
        private readonly string $id;
        private string $username;
        private int $rating;
    ) {
    }
}