Add Additional Properties ADR

[westey-m]

Motivation and Context

#2131

Description

• Add Additional Properties ADR

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.

[Copilot]

Pull request overview

Adds a new Architectural Decision Record (ADR) documenting the approach for associating AdditionalProperties-style metadata with agents/sessions in the .NET Agent Framework.

Changes:

• Introduces ADR 0017 describing options for agent/session metadata via AdditionalPropertiesDictionary.
• Documents tradeoffs across multiple API shapes (in-type property vs constructor injection vs external wrapper).
• Captures serialization considerations for session metadata.

[rogerbarreto]

We might consider adding as part of the decision adding one sample showing how this can be achieved with the (external wrapper) and potentially adding the sample as part of this PR.

[westey-m]

We might consider adding as part of the decision adding one sample showing how this can be achieved with the (external wrapper) and potentially adding the sample as part of this PR.

Let's perhaps see if people struggle with this concept first. Any sample we create around this would be very contrived, so I'd rather avoid adding something like it unless we have a clear signal.