Add more materializing extension methods.

[fiseni]

We expose the WithSpecification extension which applies the specification to any IQueryable source, refer to the docs for more details.
We might go one step further and add some convenience extension methods covering some common use cases, e.g. FirstOrDefaultAsync, SingleOrDefaultAsync, etc. We already have such an extension ToListAsync, which is actually necessary considering the PostProcessingAction. But, we can expand to that and cover more methods.

Example

Current usage:

var customer = await dbContext.Customers
    .WithSpecification(spec)
    .FirstOrDefaultAsync(cancellationToken);

Proposed:

var customer = await dbContext.Customers
    .FirstOrDefaultAsync(spec, cancellationToken);