Partial fix to detect asynchronous return types within field delegates

[Shane32]

See:

• v5 Migration problem #3046

Fixes this scenario, but few others:

Field<CharacterInterface>("hero", resolve: GetDroidByIdAsync);

Task<Droid> GetDroidByIdAsync(IResolveFieldContext<object> context)
    => _data.GetDroidByIdAsync("3");

Exceptions are thrown either during schema construction or schema initialization. Debugging will reveal the faulty line of code.

Note that we could alter this code to accept the delegate and handle it correctly. I elected not to do so, and require users to use the FieldAsync method or correct constructor.