feat(#85): allow to define a typed body

[francescovallone]

Description

Serinus now allows to define a typed body directly in the handler providing a simple API to access it.
This PR also brings a new command to the cli serinus generate models and improve the overall management of serialization and deserialization of objects by the framework.

Fixes #85

Type of change

Please delete options that are not relevant.

• New feature (non-breaking change which adds functionality)
• This change requires a documentation update

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules