Allow us customized to write value as customFields to session

[tianyingchun]

Is your feature request related to a problem? Please describe.

Is there a way to add an customField like locale to the table session

Describe the solution you'd like
allow us customized to write value as customFields to session
e.g. i want to get languageCode from ctx and store on the session to statistic How many sessions come from where?

Describe alternatives you've considered

1. add hook allow us get ctx to merge an value to customFields(locale) to Session.
2. Allow us defined custom Fields locale

Additional context

[michaelbromley]

Custom field support will be added to Session as part of #3044