Extract postgres hll namespace

What does this MR do?

It adds PostgresHll module and move PostgresHllBatchDistinctCounter into it. This change was break apart from !48778 (diffs) where PostgresHllBatchDistinctCounter was split to smaller classes. New module was required to group new smaller classes together.

This change does not modify algorithm implementation in ANY WAY it only move file into new place.

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• [-] Style guides
• [-] Database guides
• [-] Separation of EE specific content