Share optimizations between relational and non-relational providers

[ranma42]

Many optimizations are not dependent on the specific provider or whether the underlying model is relational.

It would be convenient to have a way to share them across all of the relevant targets.
Currently this is not trivial, first of all because they operate on different types (see Microsoft.EntityFrameworkCore.Cosmos.Query.Internal.SqlExpression vs Microsoft.EntityFrameworkCore.Query.SqlExpressions.SqlExpression.