adding fct_unused_staging_models

[lord-skinner]

This is a:

• bug fix PR with no breaking changes
• new functionality

Link to Issue

Description & motivation

Often when we integrate a new data source into our warehouse we stage too much data. Over time staging models go unused, but continue to be included when compiling the dbt project.

Eventually we'd like to also see unused columns in dbt_project_evaluator, but this is a good place to start.

Integration Test Screenshot

Checklist

• I have verified that these changes work locally on the following warehouses (Note: it's okay if you do not have access to all warehouses, this helps us understand what has been covered)
  • BigQuery
  • Postgres
  • Redshift
  • Snowflake
  • Databricks
  • DuckDB
  • Trino/Starburst
• I have updated the README.md (if applicable)
• I have added tests & descriptions to my models (and macros if applicable)