Add Helper handle to streamline TrackContainer construction

This MR contains changes ncessary to deply xAOD based TrackContainer. There is multiple changes involved here and there so a small guide for reviewers:

1. The multitude of handles needed for all the backends are not wrapped in MultTrajecotoryHandle and TrackContainerHandle. They are modeled to be simple to use. I.e. declaration, initialisation, and the method convertToConst that translates mutable varaiant to const varaint, registeres backend in store. I was considering the API to be: storeConstVersion(mutable) which would releive client from registering the Const containers by themselves.

There are drafts of respective Const handles. They will be needed when readoing from persistency and are not yet fully functional.

2. The surfaces support in TrackBakendCoantiner is only for transient state. It may slightly change.
3. The new containers are now in the "future" namespace to limit the scope of the test to the conversion only. Next move is to use them in CKF and
4. There are dafts of Cons

FI @cvarni @emoyse @ncalace @pagessin @wolter @adye PS. I am aware that this changeset is not perfect but would preffer to see it merged first and then add remaining fueatures and make cleanup.