Add pydantic based validation to clinvar_api models

[holtgrewe]

Is your feature request related to a problem? Please describe.
The submission schema has certain validations (e.g., minimal and maximal number of items). The schema validation results are hard to read, so using pydantic validations would improve ergonomics.

Describe the solution you'd like
Implement the validation where necessary. When a field is "required" then this is already reflected by the type annotations and the field being non-Optional.

Describe alternatives you've considered
N/A

Additional context
N/A