Utilize AsyncPageable for query return types

[timtay-microsoft]

The new APIs look like:

public virtual AsyncPageable<T> CreateAsync<T>(string query, CancellationToken cancellationToken = default)

A couple points worth discussing:

• Unlike in any of our other APIs, the AsyncPageable APIs contain the Response type that allows users to see the http request contents.
  • This is generally a good thing to let users see, but it may beg the question as to why the other APIs don't allow this same thing.
• Some code (PageableHelpers) has to be copied from Azure.Core since constructing an AsyncPageable instance is non-trivial
  • The public facing return types are the Azure.Core defined classes, though.
• The page size and continuation token can be set through the AsyncPageable itself, so we can remove the QueryOptions class that we had that served the same purpose.
  • The only real loss here is that users cannot set page size or continuation token unless they are iterating page by page. Users iterating item by item won't actually be capable of getting the continuation token, but would they still care about page size?

// Old pagination configuration using our defined QueryOptions
var options = new QueryOptions()
{
    ContinuationToken = "someToken",
    PageSize = 4
};
AsyncPageable<ClientTwin> twinQuery = serviceClient.Query.CreateAsync("some twin query", queryOptions);
await foreach (Page<ClientTwin> queriedTwinPage in twinQuery.AsPages())
{
    // some application logic
}

// Pagination configuration using AsyncEnumerable methods
int pageSize = 5;
AsyncPageable<ClientTwin> twinQuery = serviceClient.Query.CreateAsync("some twin query");
await foreach (Page<ClientTwin> queriedTwinPage in twinQuery.AsPages("continuationToken", pageSize))
{
    // some application logic
}

Once this PR is merged in, I'll apply the same changes to the DPS service client since it also has a query API

[timtay-microsoft]

This class used to be the "AsyncPageable" implementation we had locally, but now it is the "Response" implementation since the Azure.Core AsyncPageable pages all contain the Response type embedded in them

[drwill-ms]

Needs an update to migration guide too.

[drwill-ms]

Still needs an update to migration guide.

[timtay-microsoft]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[timtay-microsoft]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[timtay-microsoft]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[timtay-microsoft]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[timtay-microsoft]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[timtay-microsoft]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[timtay-microsoft]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).