.Net: Agents - Assign ChatMessageContent.Source property

[crickman]

This property is currently not-serializable and intended to take an reference to an Agent (or other) object.

It may be preferable to serialize a value for Source that is useful on deserialization.

Either way, Source property should be assigned in Agent framework.

Note: This should also be considered / justified as part of https://github.com/microsoft/semantic-kernel/issues/5828