Expand tests for BigQuery external tables

[jtcohen6]

(split off from #41)

• Figure out what's up with the ndjson source. Although staging the external table works well enough, I see this error as soon as I try to to query it: error message: JSON parsing error in row starting at position 0: Expected key. For the sake of merging BigQuery external tables #49, I've just commented out these tests.
• Will BQ add DDL support for other external sources (Cloud Bigtable, Google Drive, Cloud SQL)?

[drewmcdonald]

@jtcohen6 per your second point, yes! (full docs)

I was just doing some testing and got a sheets-backed table up and running with dbt-external-tables v0.6.2.

I haven't tried relying on schema inference or any other bells and whistles, but the stripped-down spec below works out of the box, which is really exciting!

YAML

version: 2

sources:
  - name: sheets
    database: my_project
    schema: my_dataset
  
    tables:
      - name: my_table
        external:
          location: "https://docs.google.com/spreadsheets/d/$BLAHBLAHBLAH"
          options:
            format: GOOGLE_SHEETS
            skip_leading_rows: 1

        columns:
          - name: thing_1
            data_type: STRING
          - name: thing_2
            data_type: INT64