Add additional "Commit Metadata" field to commit data structure

[timsehn]

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.

[christopherreay]

maybe add a "hasMetadata" boolean into dolt_branches, and wherever else the commit is referenced