JDBC driver JARs added when creating a new JDBC source at import time should be added as extension JARs so the connector works at runtime.

Given that Designer does not bundle any JDBC driver JARs, the need to be added by the user. Currently they are required in two places:
1. when defining an import source
2. for the connector used during preview and testing (extension jar)

The user has to deal with importing the driver JARs in two different ways to support the logical operation of Import->Model->Preview. Suggest:
a) providing a single new mechanism for adding driver JARs to the tooling so they are available for both Import and Preview/Execution, or
b) automatically add extension JAR for Preview/Execution when specifying driver JAR during import source configuration, or
c) both

In the present implementation, a novice user will have a heck of a time understanding why Preview doesn't work when they just added the JAR while configuring the source for import.