.NET: Consider adding AdditionalProperties to AIAgent

[kzu]

Being able to annotate agents with additional metadata could be very helpful in driving an app powered by them. Some scenarios:

• Agent requires user-authorized cloud storage (i.e. Google Drive or OneDrive): the app could detect this in a generic fashion by inspecting an agent's AdditionalProperties and trigger auth flow as needed.
• Agent requires payment: the app could detect this, check user's balance and trigger payment workflows as needed.
• Agent is in beta: app could hide these from lists, render them with a beta label in a catalog, etc.
• Agent has icon: ditto, for improved display in apps
• Agent is unlisted: agent should not be shown but it's available if users talk to it explicitly (i.e. via @agent mention)
• Agent is whitelist-only: agent requires an explicit opt-in by an admin for the current user

And so on.

Proposal is to just add AdditionalProperties like AgentRunOptions/ChatOptions have already, but at the agent level too.