Add default_aggregation to Field

[deepralhan26]

Overview

Add an optional default_aggregation property to Field objects to make aggregation behavior explicit and portable.

Problem

Aggregation intent is currently implied indirectly, creating gaps in portability and round-trip workflows. Moving models between systems loses this critical semantic information.

Solution

fields:
  - name: revenue
    type: number
    default_aggregation: sum

Benefits

• Explicit rather than inferred
• Improves interoperability across systems and formats
• Preserves author intent during conversion
• Gives consumers a predictable default for measure behavior

Proposed Values

sum, average, count, count_distinct, min, max, none

Backward Compatibility

Field is optional; existing models unaffected.