API: add a way to create "half-open-ended" challenges

[Pikrass]

The API allows us to create either a completely open-ended challenge, where the link needs to be followed by two players to start the game, or a completely defined challenge, where both players are known ahead of time.

I want to create a challenge link that is already accepted by one side. Same as when I click on "Play with a friend" on the Lishogi front page.

I thought I could do this by first creating an open-ended challenge, then immediately accept it with an OAuth2 token.
Unfortunately, this fails with a "Not found" error. I guess this endpoint only works for challenges that are explicitly targeted to the logged-in player.

Either the accept endpoint should work for open-ended challenges as well, or a new endpoint should be made to create a half-open challenge.

[ddugovic]

I know the PyChess community does the reverse (when a game seek is created, send a Discord webhook notification so players can discover that someone is seeking a game).

[ddugovic]

Similar upstream issue (still open at this time): lichess-org#15748