Make Views use SqlEntityStorageInterface

Follow-up from #1497374: Switch from Field-based storage to Entity-based storage → , by @plach in #248:

There's only one thing that I'd really wish to see changed: currently the views integration code of the Field module treats every class extending the DatabaseStorageController as a SQL implementation. IMHO this is an unneeded limitation, as already pointed out above by @fago: should someone need to write an alternative SQL storage controller, she would be forced to extend
the core one with all its assumptions about the underlying implementation. I'd suggest instead to provide an interface, e.g. SqlStorageControllerInterface and make DatabaseStorageController implement it. The current pseudo-private static methods could be refactored and moved onto that interface, becoming regular methods, and then the views integration code could just check whether the storage controller implements SqlStorageControllerInterface.

I realize that those methods were not made part of the public API intentionally, but I think that since there are legal use cases for needing to retrieve some details about the storage implementation (Views integration being the most obvious), we should provide an actual API that code needing to deal with SQL can properly rely on. Doing the opposite IMHO will just "authorize" people to find the most sneaky ways to circumvent the Entity Storage API. I think a SqlStorageControllerInterface marker interface would be enough for this issue, as I guess that a proper API allowing to describe any SQL table layout would not use the static methods in the current form. Refactoring those would totally be a follow-up.

Creating this issue to discuss and implement that.