Merge multi-valued field support from DistinctCountHHLMV into DistinctCountHLL

DistinctCountHLL could call `BlockValueSet.isSingleValue()` and handle a MV field, without users having to know that they need to use a different name, depending on the field's SV/MV setting.

Start by reading DistinctCountHLL and DistinctCountHHLMV, then inspect how BlockValueSet.isSingleValue() distinguishes single-valued and multi-valued fields. The work is complete when DistinctCountHLL handles both field types without requiring users to select a different function name.

Written by the indexing model from the issue text.