fix experimental message endpoint

[lucagobbi]

Description

Bugfix to adjust the current behaviour of the experimental message endpoint /message. I've also provided response_model to allow type inference on json schema autogenerated by fastapi

Related to issue #824

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas

[pieroit]

Thanks @lucagobbi !!!