.Net: Updated audio abstractions to return multiple values

[dmytrostruk]

Motivation and Context

This PR contains changes for audio abstractions to align with abstraction pattern in SK to return multiple values instead of just one to be more flexible in cases when AI connector returns multiple results (like in chat completion service with choices).

For easier usage when only one result is provided by AI connector, it's possible to use extension method that is responsible for returning single result.

Contribution Checklist

• The code builds clean without any errors or warnings
• The PR follows the SK Contribution Guidelines and the pre-submission formatting script raises no violations
• All unit tests pass, and I have added new tests where possible
• I didn't break anyone 😄