Allow using an external outbox and/or saga storage

[SzymonPobiega]

Although in theory one might mix and match persistences via UsePersistence<T, TStorageType> API, the SQL saga persistence can only work with SQL outbox persistence.

The seam between these two persistence concerns is ISqlStorageSession. If we allow registering a custom storage adapter that can return an object that implements that interface we could use external outbox persistence.

Similarly, if we allow registering a custom adapter that takes ISqlOutboxTransaction as an input we could use a custom saga storage.

[weralabaj]

I think this should be escalated for cross-repo feature consideration, since then in theory you can use any combination (or should we restrict possible combinations somehow?), e.g. Raven with SqlP or Raven with NHibernate...