api: combining fastify schemas with joi validation

[laurenzhonauer]

In the fastify documentation it is mentioned that validation libraries like joi can be "included" in the schema definition.
Currently we are doing both seperatly, by combining the joi config into the schema definition the api code could be reduced and become more readable and maintainable.

https://www.fastify.io/docs/latest/Reference/Validation-and-Serialization/#using-other-validation-libraries

[laurenzhonauer]

Idea: Completely getting rid of joi in the api.

PROS:

• Less code, as only the ajv definitions would stay
• Greater Performance (ajv outperforms joi)

CONS:

• If there are usages of joi validation, that perform some JS logic, we have to look into ajv plugins