Force the bot not to challenge an opponent

[FatFritz2]

Hi, I'd suggest adding a block list where the user can modify which player can't be challenged or match maked by the bot like lichess-bot here:

# block_list:                # List of users from which the challenges are always declined.
#   - user1
#   - user2

I mean you should add a list for the bots that can't be accepted challenges sent from them and the bots that our local bot cannot matchmaking with them in config.yml Thanks