Support SQL window functions

Both SQL Server and PostgreSQL seem to support window functions in a similar way - in at least some cases the syntax even seems to be identical. It could be nice to add support for this to EF Core.

A main question is how this would be expressed in LINQ, AFAIK there's no way to do these kinds of operations in memory.

NOTE: For an up-to-date design proposal for expressing window functions with LINQ, see https://github.com/dotnet/efcore/issues/12747#issuecomment-3152575891.