Add additional "Commit Metadata" field to commit data structure

Customers often ask where to put structured data about a Dolt commit that is generated by their application. Right now, may choose to stick structured text like JSON in the commit message and parse it out later. This is not ideal.

Instead, we could offer a JSON formatted "commit metadata" option in Dolt commit that we could store with the commit. This field would be checked for JSON compatibility at commit time. It could be accessed as a JSON column in the `dolt_log` table.

Moreover, it should be hashed into the commit id.

Start by locating the commit data structure and the dolt_log table entry point. Determine how commit fields are validated, exposed as JSON, and incorporated into the commit id; done means the metadata is stored, queryable from dolt_log, JSON-validated at commit time, and included in commit hashing.

Written by the indexing model from the issue text.