[CT-2254] dbt debug for connection testing

[jtcohen6]

Much older issue in the same vein: #3735

It should be possible to use dbt debug for connection testing. Currently, debug also includes checks for:

1. Valid profiles.yml file
2. Valid dbt_project.yml file
3. git as an installed dependency (for deps)
4. dbt can connect and submit a "debug" query to the selected profile

Of those checks, we really only need the last one for connection testing. Ideally, we'd be able to skip the other checks (perhaps by passing a flag?) and return success/error (programmatically & as CLI exit code) on the basis of the "debug" query only.

If we had to hack this today, it would be sorta possible by examining the structured events (DebugCmdOut) returned from the debug command, but this hardly feels like a stable interface.

Why dbt debug?

Why not just an interactive query (new preview command) with the query hard-coded to select 1 as id? Reasons to prefer:

• leveraging an adapter method, debug_query
• it's part of dbt-core's top-level API, and clearly the intent of debug

Questions

• How should the profile be provided? In profiles.yml, or could it be provided as a Python data structure?
• Could we avoid the need to have a dbt_project.yml? Just use a "dummy"/empty dbt_project.yml?

Acceptance criteria

• Given the contents of a profile only, validate that profile via dbt debug, and return success/error

[nssalian]

An alternative suggestion would be to add a command
dbt test_connection that would use the selected profile. I can open a separate issue to surface this alternative way.

[VersusFacit]

Just going by what we see here, I would say we communicate that dbt debug exists to support the validation of a dbt environment. Having a flag that skipped those validation steps would mirror the `git commit --no-verify` workflow. Basically, just validate that the connection makes sense. I worry that if we create a new command which under the hood is leveraging the same code, we are inviting unclear interfaces at the executable level

We already have a flag that serves as an example of what additional flag we might add to accomplish the desired behavior outlined in the issue

[VersusFacit]

With the merge of #7741 , the workflow for this in 1.6 and on will be dbt debug --connection.

Other flag skips/features may be added. See this issue.