Allow the bot to prioritize playing against humans or bots

[AttackingOrDefending]

Type of pull request:

• [ ] Bug fix
• [x] Feature
• [ ] Other

Description:

Allow the bot to prioritize playing against humans or bots. For example, if someone changes preference to human and has sort_by set to best, then the bot will prefer to play with a human even if they are rated lower than a bot. The order of opponents will be:

HUMAN A (2100)
HUMAN B (1800)
HUMAN C (1500)
BOT A (3000)
BOT B (2500)

Related Issues:

None

Checklist:

• [x] I have read and followed the contribution guidelines.
• [x] I have added necessary documentation (if applicable).
• [x] The changes pass all existing tests.

Screenshots/logs (if applicable):

[MarkZH]

One more thing. Since there are a finite set of options for this new configuration, there should be a test in validate_config() that checks that the value, if it is present, is from that set.