[source-mysql] Allow unique fields to be used if primary key is not available in incremental mode

[pedrohsroque]

Right now, Airbyte doesn't allow incremental mode if the table doesn't have a primary key, but sometimes, a unique_key field could be used for that.

It would be nice to enable the user to select the unique field or automatically select the unique field from the table so we don't have to refresh some large tables completely.

Context: we are migrating from Stitch to Airbyte, and this is a feature Stitch has.