code: def get_vendor_extension_fields(mapping):
      
      return {k: v for k, v in mapping.items() if k.startswith('x-')}
docs: 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}