support IAsyncenumerable

[ElizabethOkerio]

This PR fixes #390

Currently, If you have such a controller action method in OData AspNetCore:

 [EnableQuery]
 public IAsyncEnumerable<Customer> Get()
 {
      return _context.Customers.AsAsyncEnumerable();
 }

The request is processed asynchronously and a non-OData response is returned.

This change ensures that an OData response is returned.

The issue was, AspNetCore OData could not map an IAsyncEnumerable<> to an EdmCollectionType. To the DefaultODataTypeMapper I've added a check for an IAsyncEnumerable<>. If the collection is of type IAsyncEnumerable<> then it is mapped to an EdmCollectionType.

If we map the IAsyncEnumerable<> to an EdmCollectionType then the ODataResourceSetSerializer is used in serializing the response.

To the ODataResourceSetSerializer I've added a check for the collection type. If the type is of IAsyncEnumerable<> then we use the await foreach to loop through the resources in the collection. Otherwise, we use the normal foreach loop.

[habbes]

It doesn't appear that we are processing the IAsyncEnumerable asynchronously (i.e. using await foreach). I think that's the whole point of this PR.

[habbes]

I think you should also test with IAsyncEnumerable objects that are not IEnumerable to make sure we support that scenario as well.

[xuzhg]

[aldrashan]

Doesn't appear to work in all cases?
See OData/WebApi#2598 (comment)