Unparse a ``Sec-WebSocket-Extensions`` header.

    This is the reverse of :func:`parse_extension`.

def build_extension(extensions: Sequence[ExtensionHeader]) -> str:
    """
    Unparse a ``Sec-WebSocket-Extensions`` header.

    This is the reverse of :func:`parse_extension`.

    """
    return ", ".join(
        build_extension_item(name, parameters) for name, parameters in extensions
    )