Added IAsyncEnumerable implementations for various API calls

[JerrettDavis]

Implemented a new syntax leveraging IAsyncEnumerable to let users choose between callbacks and stream enumerations.

The new calls allowing for IAsyncEnumerable instead of an action callback are:

IAsyncEnumerable<CreateStatus?> CreateModel(CreateModelRequest request, [EnumeratorCancellation] CancellationToken cancellationToken = default);

IAsyncEnumerable<PullStatus?> PullModel(PullModelRequest request, [EnumeratorCancellation] CancellationToken cancellationToken = default);

IAsyncEnumerable<PushStatus?> PushModel(PushRequest request, [EnumeratorCancellation] CancellationToken cancellationToken = default);

IAsyncEnumerable<ChatResponseStream?> StreamChat(ChatRequest chatRequest, [EnumeratorCancellation] CancellationToken cancellationToken = default)

IAsyncEnumerable<GenerateCompletionResponseStream?> StreamCompletion(GenerateCompletionRequest request, [EnumeratorCancellation] CancellationToken cancellationToken = default);

[awaescher]

This is some premium pull request - I wanted to implement the IAsyncEnumerable by myself, too. Thanks a lot, really means much to me.