Fields that come from macros should come from macros always

[rlh1994]

Is your feature request related to a problem? Please describe.

Sometimes we refer to columns that have come from field group macros by macro, and sometimes by name. We should use the macro throughout to make it easier to make changes in the future, and in case a user overwrites the macro.

e.g.

dbt-snowplow-unified/models/sessions/scratch/snowplow_unified_sessions_this_run.sql

Lines 35 to 37 in ba00cac

	{% if var('snowplow__enable_iab') %}
	{{ iab_context_fields('ev') }}
	{% endif %}

dbt-snowplow-unified/models/sessions/scratch/snowplow_unified_sessions_this_run.sql

Lines 383 to 388 in ba00cac

	{% if var('snowplow__enable_iab') %}
	, f.iab__category
	, f.iab__primary_impact
	, f.iab__reason
	, f.iab__spider_or_robot
	{% endif %}

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