DRY around getting the projection from ShapedQueryExpression

[roji]

In many places in the query pipeline, we extract the ShapedQueryExpression.ShaperExpression (unwrapping casts), and then extract the projection from the corresponding QueryExpression; see RelationalQueryableMethodTranslatingExpressionVisitor.TryGetProjection for an example. We should DRY this, possibly adding APIs (or extensions) to ShapedQueryExpression.