Add basic variant and callset searching

This relies on adding the variantsets/search method
(https://github.com/ga4gh/schemas/pull/134)

Also adds ‘keyvalue’ as a type for easy testing (as pretty much all of
our objects have that field)
Add a NCBI dataset ID for 1kg
Merge pull request #4 from cassiedoll/ncbi-ds-id

Add a NCBI dataset ID for 1kg
Merge pull request #3 from cassiedoll/variants

Add basic variant and callset searching