Python discriminator support

[baywet]

reflection of #648 for Python
depends on #1174

TODO:

• introduce a new ParsableFactory type in the abstractions serialization package
• update parse node interface to take the new factory as parameter (getObjectValue, getCollectionOfObjectValues)
• update deserializer method generation to pass the factory for the properties + update unit tests
• add factory to request adapter interface and implementation as parameter, and pass the factory to the parse node methods
• update request executor generation to pass the factory to the request adapter send method + update unit tests
• bump abstractions, http and serialization packages
• implement factory method generation + unit tests
• use refiner AddDiscriminatorMappingsUsingsToParentClasses to add reference to types in discriminator mapping