ImportVariants: Creates variant data by asynchronously importing the provided information.
The variants for import will be merged with any existing data and each other according to the behavior of mergeVariants. In particular, this means for merged VCF variants that have conflicting info fields, some data will be arbitrarily discarded unless otherwise specified in the InfoMergeConfig field of ImportVariantsRequest. As a special case, for single-sample VCF files, QUAL and FILTER fields will be moved to the call level; these are sometimes interpreted in a call-specific context. Imported VCF headers are appended to the metadata already in a variant set.