Publicly export SQLBackend

[NickCrews]

Not every backend supports create_view(), only SQLBackends. I have a function with signature def make_view(backend, t: ibis.table). I want to be able to annotate the type of backend param. I can, but have to import using the not-explicitly-exported from ibis.backends.sql import SQLBackend method.

Can we make it so I can import it from a stable place? Perhaps Simply ibis.SQLBackend? IDK if it is important enough to go at the top-level.

This also brings up the larger question, which I'm not sure of the answer: does the simple dichotomy of SQL/not-SQL really cleanly partition all backends into two groups? Or would need to do some capability based typing like with protocols, eg class SupportsCreateView(typing.Protocol): def create_view(...): ...? This isn't a huge priority, if it is a huge can of worms then we could definitely punt.