Feature request: let extract-db-metadata only extract the definitions of the relations used in a mapping

[bcogrel]

Suggested in #570 by @Aklakan

Currently, the command extracts all the DB metadata. By providing a mapping file as an optional parameter, the command could extract only the definitions of the relations used in this mapping.

[jindrichmynarz]

Even restricting the scope of extract-db-metadata to a particular database schema would be helpful. For example, the database user might not have permissions to access all database schemas in which case extract-db-metadata fails.