.Net Agents - Support streaming for ChatCompletionAgent

[crickman]

Motivation and Context

Enables streaming invocation of ChatCompletionAgent for use in no chat (single agent) mode.

Description

• Add ChatCompletionAgent.InvokeStreamingAsync
• Add streaming sample
• To be incorporated as part of AgentChat streaming framework (i.e. this is an initial component)

Contribution Checklist

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