feat: allow to define a typed body

[francescovallone]

Description

To better use the request body in Serinus, the framework should allow to define a typed body.

Requirements

• [x] Define the interface for a typed body.
• [x] Update Tests.
• [x] Update Documentation.

[francescovallone]

The proposal is the following:

• [x] Add an option to the command generate for the models
• [x] Remove the config.yaml file and use the pubspec.yaml to manage all the serinus things
• [x] Add a ModelsProvider to ensure the models are provided correctly
• [x] Integrate json_serializable, dart_mappable, freezed and allow to define an own serialization strategy.