Add ONVIFResponse wrapper for zeep objects with serialization support

[kaburagisec]

This PR will allow us to convert ONVIF operation responses containing zeep objects into dicts using the to_dict() helper function.

Example usage:

from onvif import ONVIFClient

client = ONVIFClient('192.168.1.100', 80, 'admin', 'password')
device = client.devicemgmt()

# usual usage (zeep object)
info = device.GetDeviceInformation()
print(info.Manufacturer)
print(info.Model)

# convert to dict with to_dict() function
info_dict = info.to_dict()
print(info_dict)
# Output: {
#     'Manufacturer': 'ACME',
#     'Model': 'Camera-1000',
#     'FirmwareVersion': '1.0.0',
#     'SerialNumber': '12345',
#     'HardwareId': 'v1.0'
# }

# another examples
capabilities = device.GetCapabilities()
caps_dict = capabilities.to_dict()