Add validation rules to MutationsByLineage

[flaneuse]

Refactor MutationsByLineage 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. Add validation rules for the parameters:

• location_id: str
• mutations: str
• pangolin_lineage: str
• frequency_threshold: float, min = 0, max = 1