[WIP] Extra SQL where constraints for Views and DataSources middleware

[boazy]

@ronen could you please review this PR?

This all goes towards what we discussed in:
SchemaPlus/schema_plus_views#16

I ended up replacing the default Rails implementation, so perhaps it's better to move this code to a new, separate gem and use an implement Middleware.

[boazy]

Oh, and checks obviously fail because it relies on the new methods added to schema_plus_compatibility.

[ronen]

I ended up replacing the default Rails implementation, so perhaps it's better to move this code to a new, separate gem and use an implement Middleware.

Yeah I think it should be separate--schema_plus_core should stick to its philosophy of providing an API but not adding new features or changing implementation or behavior.

Only question is which gem should it go in... schema_plus_data_sources? Include it with an existing gem? I notice BTW that the existing schema_plus_tables has no use in AR 5.0; so schema_plus tables can be retired, or maybe it can become the home for this middleware?

[boazy]

This change actually doesn't affect default behavior, but it might do so in the future - if the rails implementation is changed.
But seeing how Rails 5 dealt with Connection#tables and Connection#data_sources, I don't expect they'll surprise us with a function that suddenly returns completely different results without having a deprecation warning first.