SCM: Remove PageCollection types in favor of using CollectionResult

[annelo-msft]

Background

Given feedback from partners that SCM PageCollection<T> and related types were too complex for the convenience layer, we have reworked the client patterns to simplify usage for the simplest use case of paginated service methods -- i.e. the return type now implements IEnumerable<T>, which means that strongly typed models in a paginated collection can now be enumerated directly. This better aligns the SCM client pattern for pagination with the Azure client pattern as well. One side effect of this is that the implementation of a complex scenario like rehydrating a collection becomes more complex, but this seems like a good tradeoff.

What's in this PR?

• This PR removes PageCollection<T> and related types in favor of new APIs on CollectionResult<T> and AsyncCollectionResult<T>
• It adds two new protocol-method return types CollectionResult and AsyncCollectionResult that the convenience-layer return types now inherit from
• CollectionResult<T> no longer inherits from ClientResult, but instead enables access to the sequence of service responses that deliver the collection items via a new GetRawPages method on the CollectionResult base type
• Since it is preferrable that ClientResult represents a single HTTP response, this PR removes its parameterless constructor and moves its SetRawResponse method onto OperationResult, which is the only type that now needs it to update the value returned from GetRawResponse
• Given the changes to the design of the collection result types, tests have been restructured and simplified, and reference implementations provided to show how clients might implement internal subtypes of CollectionResult and related types for both paginated service endpoints and streaming responses.

[azure-sdk]

API change check

APIView has identified API level changes in this PR and created following API reviews.

System.ClientModel

[annelo-msft]

/azp run net - openai - ci

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