Identify vendor extension fields and extract them into a new dictionary.
    Examples:
        >>> get_vendor_extension_fields({'test': 1})
        {}
        >>> get_vendor_extension_fields({'test': 1, 'x-test': 2})
        {'x-test': 2}

def get_vendor_extension_fields(mapping):
    """
    Identify vendor extension fields and extract them into a new dictionary.
    Examples:
        >>> get_vendor_extension_fields({'test': 1})
        {}
        >>> get_vendor_extension_fields({'test': 1, 'x-test': 2})
        {'x-test': 2}
    """
    return {k: v for k, v in mapping.items() if k.startswith('x-')}