Add validation rules to PrevalenceByAAPositionHandler

[flaneuse]

Refactor PrevalenceByAAPositionHandler to add validation:

1. As before, create a copy of the handler to enable easy testing between the new version of the handler and the existing one
2. Refactor the structure of the parameters to be a BioThings-style kwargs format
3. change the parameter location to location_id
4. Add validation rules for the parameters:

• pangolin_lineage: str
• location_id: str
• name: str