Add GHA for CI for database adapters

[kwigley]

No description provided.

[jtcohen6]

Could this be as simple as:

• Each plugin's CI installs the latest version of dbt-core (from develop branch in this repo)
• We schedule each plugin's CI to run once per night

Downside of this approach is, we wouldn't have a centralized report of failures in this repo—but we would get the notification from the plugin repositories, and we'd still have the uploaded artifacts

[jtcohen6]

Integration tests run nightly in the three main plugin repos, and they install the latest changes from (HEAD version of) dbt-core.

We still need to:

• figure out how we're reliably notified by failures
• do the same for dbt-spark (Repo housekeeping pre-v1 dbt-spark#227)

[jtcohen6]

This is 95% complete. We still need:

• notification on failures
• complete dbt-spark cutover to GHA

I'm going to close this issue in favor of new issues to tackle both of the above.