[CT-297] get_response should always return AdapterResponse

[jtcohen6]

Split off from #4433:

In v1.1.0, let's enforce that the get_response abstract method must return an AdapterResponse

This will allow us to clear up some tech debt. At its very simplest, an AdapterReponse can just look like the one in dbt-spark:

    @classmethod
    def get_response(cls, cursor) -> AdapterResponse:
        return AdapterResponse(
            _message='OK'
        )

Ideally, of course, an adapter has more interesting information to pass along (number of rows affected, bytes processed, relevant query/job ID, etc).

Let's do this for v1.1.0, with clear communication ahead of time for adapter maintainers, since this does constitute a breaking change.

[MartinNowak]

Sounds great, we've actually forked dbt to hack in logging of BigQuery job links.

[jtcohen6]

@MartinNowak Sounds like you might be interested in dbt-labs/dbt-bigquery#92!