.NET: Consider adding AdditionalProperties to AgentRunOptions

[kzu]

When building pipelines of behavior, I found the ChatOptions.AdditionalProperties very useful to place extended info a given cross-cutting concern provides.

A similar pattern in AgentRunOptions would be very helpful. See: https://github.com/dotnet/extensions/blob/09472076b1c0d53822d9ce085d9df408286bb867/src/Libraries/Microsoft.Extensions.AI.Abstractions/ChatCompletion/ChatOptions.cs#L222-L223

[kzu]

I've sent a PR, hopefully this will make it sooner than later?