Pluggable jdbc connection factory

The ability to provide a custom connectionFactoryProvider via JDBCOptions so that JdbcUtils.createConnectionFactory can produce a custom connection factory would be very useful. In our case we needed to have the ability to load balance connections to an AWS Aurora Postgres cluster by round-robining through the endpoints of the read replicas since their own loan balancing was insufficient. We got away with it by copying most of the spark jdbc package and provide this feature there and changing the format from jdbc to our new package. However it would be nice  if this were supported out of the box via a new option in JDBCOptions providing the classname for a ConnectionFactoryProvider. I'm creating this Jira in order to submit a PR which I have ready to go.