Types to represent user input request and response for additional details

[SergeyMenshykh]

This PR adds the AdditionalDetailsRequestContent class to represent requests from AI agents for additional details, and the AdditionalDetailsResponseContent class to represent corresponding responses to those requests.

For now, the pair of classes will be used by the A2A AI agents to indicate that additional details need to be provided to the agents. The request from and response to the A2A agent can be represented by TextContent, UriContent, or DataContent AI content types.

Microsoft Reviewers: Open in CodeFlow