Jackson => Gson (2/2)

[mathieucarbou]

This PR:

1. removes Jackson from platform and uses Gson instead.
2. Removes any Json-related code from these modules:
   • Nomad
   • Sanskrit
   • iNet
   • Struct
   • DC model
   • DC API
   • DC Server API

No change in the communication format, test ran against a previous platform kit using Jackson.

Some important notes:

• Gson works completely differently than Jackson. It is field-based and reflection-based
• Gson is really SIMPLE. It does its best to prevent user from serialising when it knows it won't be able to deserialise.
• As a consequence, Gson does not support polymorphism
• Gson does also not support class loading and mapping Class objects
• Gson also does not support raw / wildcard types since it cannot deserialise
• Gson has no mixin support
• Gson factory order is important
• Gson is strongly types and makes a string difference between declared types (used to select type adapters) and runtime types

All these missing features had to be added. So a lot of code was written to cope with missing Gson features, and also allow class loading and unknown type deserialisation, through a mechanism where we only specify the type names we allow for security reasons.

Gson is really easy to use. The *Module classes in different modules are stripped down compared to Jackson ones and writing custom type adapters is easier too.

[mathieucarbou]

moving to draft - refactoring of how the GsonModule works internally.