Ability to provide an existing DbTransaction to the session builder

[danielmarbach]

Currently it is already possible to provide an existing connection to the session builder with the following approach

var sessionBuilder = sessionFactory.WithOptions();
sessionBuilder.Connection(existingDbConnection);
var session = sessionBuilder.OpenSession();

this is great to be able to pass externally managed connections into a new session. In NServiceBus we have a SqlServer transport that manages under certain transport transaction modes connections and transactions for every incoming message. For those scenarios it would be great if we could customize the session with an already existing transaction too by doing for example

var sessionBuilder = sessionFactory.WithOptions();
sessionBuilder.Connection(existingDbConnection, existingDbTransaction);
var session = sessionBuilder.OpenSession();

as of now it seems this would only be possible by doing complicated decoration of session factory and transaction factory to achieve the same.