[tools] add support for structured responses

[honzasterba]

Add new response class to be able to return structuredContent as part of tools response in a backwards compatible way

Motivation and Context

Structured responses are an established part of the spec and the SDK should support it.

How Has This Been Tested?

Add new tested and tested with internal MCP server implementation.

Breaking Changes

None

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update

Checklist

• I have read the MCP Documentation
• My code follows the repository's style guidelines
• New and existing tests pass locally
• I have added appropriate error handling
• I have added or updated documentation as needed

[dieter-medium]

Nice job!

Does this not also require an 'outputSchema'?

[honzasterba]

output schema is optional for structured output, for now I just added support for the response field