Feature/add debug query method

[zmac12]

resolves #2751

Description

In the base debug task, there was a hard-coded test query that wouldn't work in some databases (e.g. Oracle). This new debug_query method on the base adapter allows plugin authors to specify their own debug query.

Checklist

• I have signed the CLA
• I have run this code in development and it appears to resolve the stated issue
• This PR includes tests, or tests are not required/relevant for this PR
• I have updated the CHANGELOG.md and added information about my change to the "dbt next" section.

[zmac12]

First time committer to any open source project here! I believe my change to debug.py needs a parameter in the new method, but I'm unsure what it should be. If any of you had a suggestion as to what it needs to be I'll revise the PR and open it again. Thanks!

[jtcohen6]

@zmac12 I think you're on the right track! debug_query should go in core/dbt/adapters/base/impl.py rather than connections.py, and in the general case I it can be as simple as:

def debug_query(self):
    self.execute('select 1 as id')

Then plugin authors can override it by reimplementing the adapter method for themselves, e.g. in core/dbt/adapters/oracle/impl.py:

def debug_query(self):
    self.execute('select 1 from dual')