Improve API validation so openapi.yaml file utilizes schemas from API model Python classes

[Kami]

We need to improve / fix API validation so all the inputs are correctly validated based on the schema defined in openapi.yaml / API model.

Right now we have schemas from API models copy and pasted to openapi.yaml file and that's a maintenance nightmare (things will get out of sync, etc.).

We need to decide which place we want as an authoritative source (either API model Python classes or openapi.yaml file) and then write a generator which either writes schema to openapi.yaml file based on API model class definitions or vice-versa.

See #3573 (comment) for context.

[Kami]

I would personally prefer to use API model as authoritative source, because:

• We already have a process which generates final openapi.yaml file based on the Jinja template definition.
• API model schemas definitions are already used in other places - e.g. when content comes into the system outside API (via st2-register-content).

If we want with the other approach, we would then have multiple files which are auto-generated and I would prefer to limit auto-generation to openapi file.