ClientModel: AsyncResultCollection<T> and SSE event collection implementation

[annelo-msft]

Overview

System.ClientModel-based clients need to be able to return collections of values from service methods, for example, for endpoints such as OpenAI's streaming chat completions where incremental updates are delivered sequentially via SSE streams. In .NET, we represent such collections as async streams.

What's in this PR

This PR adds sync and async abstractions for collections of results, ResultCollection<T> and AsyncResultCollection<T>. It also adds an internal implementation of an SSE parser and event enumerable that will likely be replaced by dotnet/runtime#98105 when it becomes available. The internal implementation is provided here to enable test coverage and review, as well as illustrate how clients would implement the abstractions to support both convenience method return values (see MockSseClient.cs) and factory-method helpers for use with protocol methods (see MockSseClientExtensions.cs).

Additional notes

Given our timelines, the current implementation uses System.IO.StreamReader, which converts SSE lines to UTF-16. This adds a performance cost in cases where we convert the data payload back to UTF-8, e.g. for use with ModelReaderWriter.Create APIs. It is beyond the scope of this PR to provide UTF-8 stream reading capabilities, but part of our longer term plans for third-party clients.

Also: This builds on a great deal of prior work on the SSE reader by @KrzysztofCwalina and @trrwilson.

Addresses #42284

[azure-sdk]

API change check

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System.ClientModel