.Net: feat/declarative agents

[baywet]

fixes #9848
This is a draft PR, at this point it's not ready for a full review.
I'd like feedback on the location of the new extension method, and on the fact that I had to add a reference to the agents project.
Here is the rationale for placing it here: it needs access to the copilot agent plugins extension method and a few other types.
Here is the rationale for adding the reference to the agents project: it needs access to the kernel agent type.

[baywet]

Here is a recap from a few internal discussions. We don't need to make any decisions right now, but hopefully it can be used in later decisions.
The extension method is currently creating the agent object, and most properties are init only to avoid mutating the object and causing side effects.
This is problematic because the end user might want to apply additional configuration to the agent in addition to what was loaded from the declarative agent. A good example of that is using a history reducer for example.
There are a couple of ways we could offer that:

• Adding parameters for the user to pass all things that could be properties on the agent object: This is going to provide a poor developer experience and might even require reflection because the agent type is generic.
• Relaxing the properties to provide setters instead of init: allowing the application developer to mutate things after the object is created might have unanticipated side effects.
• Implementing copy constructors, allowing people to pass a "configuration object": this would impact the whole inheritance tree and add additional maintenance burden.
• Implementing a builder pattern, adding an additional builder argument so both the application developer and the extension method can call into the builder to configure the agent: possible, the extension method overload would have to call the "build" method. The build method probably needs to be async since some agent need to be initialized async
• Implementing a factory pattern, adding an additional factory argument to an overload: similar to builder, matter of preference and consistency. Might require relaxing some init to set for both sections of the code to configure the agent.