Improve startup logs for `opi`

[gcapizzi]

When starting opi, this is what the user sees:

{"timestamp":"...","level":"info","source":"handler","message":"handler.opi-connected","data":{}}

This is not great, as it's not clear what handler and opi-connected mean. It would be great to see something like:

{"timestamp":"...","level":"info","source":"api","message":"api.started","data":{}}