Introduce a ToAsyncQueryable method

[HappyNomad]

I have a situation in which I must asynchronously execute an IQueryable in a standardized way, regardless of whether it's against a database or in-memory objects. So I need a ToAsyncQueryable method that operates on IQueryable. The building blocks for this feature -- IAsyncQueryable, AsyncQueryable generated extension methods, and IAsyncQueryProvider -- are already present in the System.Interactive.Async.Providers assembly.

The AsyncQueryable class's AsAsyncQueryable method operates on IAsyncEnumerable, which means I must first call ToAsyncEnumerable on my IQueryable. This defeats the purpose since, when executing against a database, the entire enumerable is read before I even have the chance to utilize the IAsyncQueryable result.

[akarnokd]

@bartdesmet Since Ix.Async is out now, is this thing to be worked on or out-of-scope?