Allow columns to be renamed regarding conversion events

[ryan-systematik]

Description & motivation

Current Issue:

Build of stg_ga4__page_conversions model fails when the value set for conversion_events is a special character that does not correspond to a column name.

Soluton:

Define a new optional variable conversion_event_column_renamers whose key matches conversion_events and assign the column name renamed at query generation time.

Checklist

• I have verified that these changes work locally
• I have updated the README.md (if applicable)
• I have added tests & descriptions to my models (and macros if applicable)
• I have run dbt test and python -m pytest . to validate existing tests