Support for custom sorting order

[aelfric]

Using the JDBC source connector, we have a use-case where we have a composite primary key comprised of an ID and a version. When an update is made, previous rows are marked as inactive and new row with an incremented version is created. All effected rows will be updated with a modification timestamp. By default, when using the timestamp strategy for the JDBC source, the query that is build appends ORDER BY timestamp_column to the query. We would like this to be more deterministic and have the ordering depend first on the version column. Would a use-case like this be a valid case to open a PR?