.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 😄

[markwallace-microsoft]

Naming needs so work, using Extensions twice in the same name seems like a bad code smell

[markwallace-microsoft]

Do we need Extension in the name?

[SergeyMenshykh]

The property seems a little bit broad in meaning and might be a limiting factor later when there is a need to add another type of state extension. A narrower name could be ConversationStateExtensions.

I am also entertaining the idea that the concept of context might be a better alternative here instead of state. It seems that context is something more flexible and fluid compared to state. The context can eventually be saved as a state or might not.

Another idea is to not use either context or state, and instead have just extensions and an extensions manager. This way, it will be up to the specific implementation of an extension to decide how to handle the On* methods calls: whether to persist messages to the state, send them for analysis, telemetry, fact extraction, etc.