Redo our pruning implementation

[roji]

Our current pruning logic currently only operates on SelectExpressions which are present in the Tables list of other SelectExpressions; it notably doesn't handle subqueries embedded e.g. in the predicate.

For example, given the following entity type:

public class Blog
{
    public int Id { get; set; }
    public string? Name { get; set; }
    public int Unneeded { get; set; }
}

... and the following query:

_ = ctx.Blogs
    .OrderBy(b => b.Id)
    .Take(10)
    .Take(5) // Causes pushdown
    .Select(b => b.Name)
    .ToList();

... the Unneeded column gets properly pruned from the projection:

SELECT TOP(@__p_1) [t].[Name]
FROM (
    SELECT TOP(@__p_0) [b].[Id], [b].[Name]
    FROM [Blogs] AS [b]
    ORDER BY [b].[Id]
) AS [t]
ORDER BY [t].[Id]

But if you place the same thing inside a subquery:

_ = ctx.Blogs
    .Where(b => ctx.Blogs
        .OrderBy(b => b.Id)
        .Take(10)
        .Take(5) // Causes pushdown
        .Select(b => b.Name)
        .First() == "foo")
    .ToList();

... that yields the following SQL, which does project Unneeded:

SELECT [b].[Id], [b].[Name], [b].[Unneeded]
FROM [Blogs] AS [b]
WHERE (
    SELECT TOP(1) [t0].[Name]
    FROM (
        SELECT TOP(5) [t].[Name], [t].[Id]
        FROM (
            SELECT TOP(10) [b0].[Id], [b0].[Name], [b0].[Unneeded]
            FROM [Blogs] AS [b0]
            ORDER BY [b0].[Id]
        ) AS [t]
        ORDER BY [t].[Id]
    ) AS [t0]
    ORDER BY [t0].[Id]) = N'foo'

The reason for this is that the pruning logic isn't implemented as a proper visitor which visits the entire tree; instead, SelectExpressionPruningExpressionVisitor (in postprocessing) calls SelectExpression.Prune; this recurses into Tables, but not into the other parts of the query.

We should extract the pruning logic out to SelectExpressionPruningExpressionVisitor, taking care with SelectExpression private state etc. Note #31049 (comment) which is similar, abuot allowing providers to add other pruning logic.