ITextCompletion Multiple Results (Incl. Streaming)

[rogerbarreto]

Motivation and Context

Potential to get multiple results for the same prompt with and without streaming support.

Resolves #526 Closes #526

Description

This PR introduces abstractions and the capability to get multiple results for the same prompt using the same request including streaming support.

Proposed:

• Added 2 result abstractions ITextCompletionResult and ITextCompletionStreamingResult
• Changed current base ITextCompletion interface with the multiple results abstraction capability
• Added 2 extensions methods to ITextCompletion to keep it retro compatible / avoid major breaking changes
• Added concrete implementation to: Azure OpenAI / Open AI / Hugging Face completion impl.
• Added Samples (with and without streaming) how to use multiple results

Breaking potential:

• Any custom completion implementation will need to implement the new methods in ITextCompletion interface with the ITextCompletionResult/ITextCompletionStreamingResult abstractions. Pretty straightforward and example was added in CustomLLM sample.

Contribution Checklist

• [x] The code builds clean without any errors or warnings
• [x] The PR follows SK Contribution Guidelines (https://github.com/microsoft/semantic-kernel/blob/main/CONTRIBUTING.md)
• [x] The code follows the .NET coding conventions (https://learn.microsoft.com/dotnet/csharp/fundamentals/coding-style/coding-conventions) verified with dotnet format
• [x] All unit tests pass, and I have added new tests where possible
• [x] I didn't break anyone :smile: