.NET: Add ability to mark the source of Agent request messages and use that for filtering

[westey-m]

Motivation and Context

This is addressing feedback that messages should be annotated with their source, rather than split by type when passed to the AIContextProvider and ChatHistoryProvider.

#2542

Description

• Add AdditionalProperties value to ChatMessage to annotate where it came from for certain cases
• Treat all chat history, ai context provider and user input as request messages, and filter using the new property.

Contribution Checklist

• The code builds clean without any errors or warnings
• The PR follows the Contribution Guidelines
• All unit tests pass, and I have added new tests where possible
• Is this a breaking change? If yes, add "[BREAKING]" prefix to the title of the PR.