Add tags, freshness, source description and description to stg_sources model

[alexfetterman]

This is a:

• [ ] bug fix PR with no breaking changes
• [x] new functionality

Link to Issue

https://github.com/dbt-labs/dbt-project-evaluator/issues/431

Description & motivation

I wanted to be able to query data about source tags, the freshness configuration of each source, and souce / source table descriptions via the stg_sources model in the dbt Project Evaluator package.

This PR

1. Adds the four fields to the stg_sources model.
2. Adds the four fields to the get_source_values macro.
3. Adds the four fields to the int_all_graph_resources model.

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)