Advanced aggregates - (V2 - Multi-entity/grouping depth)

# Description
Extend aggregates to multi-link scenarios, having, and complex groupings.

# Motivation
Closer parity for advanced reporting queries.

# Acceptance Criteria
  - Support for having; multiple groupby columns; link-entity attributes in aggregates where valid.

# Suggested Tests
  - Complex FetchXML aggregate samples; golden expected outputs.

# Dependencies

#359 