Currently we use ConstantExpression of EntityQueryable as query root.
EntityQueryable is an implementation of IQueryable which wraps current expression trees and allows it to be enumerated using IQueryProvider.

• IQueryProvider is not needed during expression tree processing in anyway. It is a bootstrapper to execute query.
• By introducing query root as expression we can streamline our implementation since we deal with ExpressionVisitors everywhere. (We pattern match constant of entity queryable everywhere
• Allows derived query roots which can be visited by individual visitors also.

Split off from #18923