Add tox + GHA

[jtcohen6]

Issue this PR Addresses/Closes

Closes #318

Summary of Changes

• Add tox with Postgres as target (dbt_integration_postgres)
• Add GitHub Action for running integration tests against Postgres via tox
• [Nice to have] Add dev-requirements.txt to install tox and latest dbt-core + dbt-postgres for local development/testing
• TODO: Set up integration tests for other adapters by adding to tox.ini + supported_adapters.env + GH workflow.
  • To be compatible with dbt Labs' continuous env vars need to match the naming convention described in https://github.com/dbt-labs/dbt-core/discussions/10668.
  • This should enable you to fully remove integration tests running on CircleCI. (I have left integration_tests/ci/profiles.yml in place so that CircleCI can continue running unchanged in the meantime.)

dbt_integration_postgres: commands[0] /Users/jerco/dev/product/packages/dbt-expectations/integration_tests> dbt deps --target postgres
15:00:17  Running with dbt=1.9.0-a1
15:00:17  Installing ../
15:00:17  Installed from <local @ ../>
15:00:17  Installing calogica/dbt_date
15:00:18  Installed from version 0.10.1
15:00:18  Up to date!
dbt_integration_postgres: commands[1] /Users/jerco/dev/product/packages/dbt-expectations/integration_tests> dbt build --target postgres
15:00:18  Running with dbt=1.9.0-a1
15:00:19  Registered adapter: postgres=1.9.0-a1
15:00:19  Unable to do partial parsing because saved manifest not found. Starting full parse.
15:00:20  Found 12 models, 157 data tests, 685 macros
15:00:20  
15:00:20  Concurrency: 5 threads (target='postgres')
...
15:00:22  Done. PASS=169 WARN=0 ERROR=0 SKIP=0 TOTAL=169
  dbt_integration_postgres: OK (5.22=setup[0.03]+cmd[1.43,3.76] seconds)
  congratulations :) (5.28 seconds)

Why Do We Need These Changes

As explained in #318, by setting up tox as a standard entry-point, the Core development team at dbt Labs can include dbt-expectations integration tests (along with other popular dbt packages) as part of our continuous development & delivery of dbt.

Reviewers

@clausherther