Add runWithConnection(), callWithConnection()

[gavinking]

We should add the ability to do work against a Vert.x SqlConnection by adding operations like EntityManager.xxxWithConnection().

[DavideD]

We decided not to do this:

• We don't want to expose the Vert.x client in the API
• When using Quarkus, it's possible to inject the pool and get a connection: https://quarkus.io/guides/reactive-sql-clients#multiple-datasources

We can re-open this if we change mind