Inform bot of GitHub id

[pmonks]

When I am interacting with the bot, I want to be able to provide my GitHub identity to it, so that when the bot is performing GitHub actions on my behalf that identity can be included (e.g. via an @mention) in the bot's commentary.

This would require storage of a Symphony identity (potentially email address & pod id) ↔︎ GitHub id mapping, which is the kind of thing the ssf-metadata repository supports.