.Net: Initial check-in for the A2A Agent implementation

[markwallace-microsoft]

Motivation and Context

Description

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 😄

[r4hulp]

Would love to see this in action. 👀

[hansmbakker]

@markwallace-microsoft great that you're working on this, I'm eager to try it!

Are there any major things that people should be aware of if they want to try it?
When would this be likely to be merged to main?