.Net: Add StateExtensions ADR, Abstractions, update AgentThread and add Unit tests.

[westey-m]

Motivation and Context

#10100
#10712

Description

This is the first PR of a number of PRs and adds

1. A base abstraction for creating a conversation state extension
2. A class to manage multiple extensions
3. Updates to the AgentThread to use extensions
4. Unit tests
5. ADR document

In follow up PRs I'll add:

• Updates to Agents to use extensions on AgentThread
• Individual Extension implementations

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 😄