OGS: optional chat message (configurable content) on connect to warn opponent

[CmdrDats]

on connect, it would be nice to send a chat message to let the opponent know that we're using igoki so that they're aware.

This should definitely be opt-in though - default message could be something like:

"Hi, I'm playing the game using a physical board and igoki to connect, so my interface is limited, and I may miss chat or undo requests. Let's have a good game!"