[http-python-client] enhance additional properties support for python dpg model

[tadelesh]

current behavior:
we treat all additional properties as raw data when serialization/deserialization, no extra typing/format info is taken into account. so, user could only use them with raw data. if user uses model/datetime/timedelta etc. type for additional properties, the serialization logic could do the serialization correctly but only with default behavior (e.g., datetime to rfc3339 format).
expected behavior:
serialization: support format etc.
deserialization: support deserialize with typing info (model type/datetime format/duration format etc).