Add support for UnInvertedField based faceting to FacetComponent

The new JSON Faceting API supports building an UnInvertedField to do faceting which would be beneficial to add to Solr FacetComponent. This would provide additional options to implementors to choose the appropriate method of faceting for their particular use case.

Summary

added facet.method=uif which invokes UnInvertedField (top level data structure). facet.prefix and facet.contains are not supported. there are some jiras cover this gap.
Note: field should be multivalued=true to get handled with uif, otherwise it silently goes to DV. Watch fieldValueCache stats while evaluating!