Implement GraphQL queries for v4 variants

[nawatts]

Add v4 as an option for dataset arguments.

• graphql-api/src/graphql/types/dataset-id.graphql
• graphql-api/src/datasets.js

Add GraphQL queries for v4 variants.

• graphql-api/src/queries/variant-datasets
• graphql-api/src/queries/variant-queries.js