[CT-236] Design for new version of dbt-adapter-tests

[leahwicz]

We want to revamp the dbt-adapter-tests framework. This issue is to do the discovery work and come up with a design on how we want to do the following:

• The new framework should take the place of [pytest-dbt-adapter](https://github.com/dbt-labs/dbt-adapter-tests), which is currently used by a few dozen community- and vendors/partner-maintained adapter plugins
• “Standard” functionality comes out of the box, so new adapter developers can know whether their adapter works in the standard ways dbt would expect
• It’s easy to extend / add tests for custom, database-specific functionality (e.g. different incremental strategies, model configs)