[FEAT] Feasibility to extend the record schema to accommodate custom fields

[My3VM]

Feature Description
Currently the record schema has fixed set of fields, which are sufficient for generic use case, it would be convenient to have extended attributes like thread_id, session_id etc which makes it better fit for end to end solutioning.

Reason
As a workaround, I have tried subclassing Record class to accommodate additional fields, but adding those new fields to migrations has been the challenge as they are fixed in the first_revisions.py.

Importance of Feature
Very high for promoting trulens to production grade solution.