Expose DOM transactions from DataBroker/TransactionChain

There are a number of components which use mixed BI/BA datastore access. It seems it would be nice if BA constructs could optionally, when realized through mdsal-binding-dom-adapter, expose specialist methods to allocate underlying transactions.

These should be exposed as add on services, so they can be discovered via MountPoint and OSGi SR, as well as other DI frameworks.