Add content to represent service-side actions

[jozkee]

Introduce ServiceActionContent and implement it in CodeInterpreter and McpServerTool contents.

Naming considerations:

• HostedServiceCallContent - Not nice for Result contents, and may be confusing with FunctionCallContent.
• HostedServiceSideContent - "Side" is uncommon in the codebase.
• HostedServiceContent - If we use this name, HostedFileContent should also extend it which escapes the scope of "content about server side actions".
• ServiceActionContent - Breaks the Hosted prefix we've been using but I like the shortness; con: "Action" may be too broad.

Contributes to https://github.com/dotnet/extensions/issues/6492#issuecomment-3603938270.

Microsoft Reviewers: Open in CodeFlow