feat: add support for structured content for MCP

[hallvictoria]

fixes: Azure/azure-functions-python-worker#1825, Azure/azure-functions-python-worker#1836, Azure-Samples/remote-mcp-functions-python#38

Examples:

# ============================================================================
# Example 1: Returning ResourceLinkBlock - defined class
# ============================================================================
@app.mcp_tool()
def get_functions_logo(context: func.MCPToolContext) -> func.ResourceLinkBlock:
    """Returns the Azure Functions logo as a resource link."""
    return func.ResourceLinkBlock(
        uri="file://logo.png",
        name="Azure Functions Logo",
        mime_type="image/png"
    )

# ============================================================================
# Example 2: Structured Content with @mcp_content decorator - custom classes
# ============================================================================

@func.mcp_content
@dataclass
class Snippet:
    """Code snippet with metadata.
    """
    name: str
    content: Optional[str] = None

# In-memory snippet storage
snippets_cache = {}


@app.mcp_tool(use_result_schema=True)
def get_snippet(name: str) -> Optional[Snippet]:
    """
    Retrieves a code snippet by name.
    Returns structured content automatically because Snippet has @mcp_content.
    """
    if name in snippets_cache:
        return Snippet(name=name, content=snippets_cache[name])
    return None