Add variantName parameter to variants endpoint

[aliceboizet]

Add support for the variantName query parameter to the Variants endpoints to allow filtering variants by their name.

Proposed changes
Add an optional query parameter to the following endpoints:

Endpoint	Parameter name	Type	Description
GET /variants	variantName	string	The human readable name of a variant
POST /search/variants	variantNames	array of string	A list of human readable variant names