feat: run BQ queries as single statement

[betodealmeida]

SUMMARY

When running multiple statements, BigQuery will not keep track of state during the lifetime of the cursor. This means that queries like this one will fail:

DECLARE d DATE DEFAULT CURRENT_DATE();
CREATE TEMP FUNCTION Add(x INT64, y INT64) AS (x + y);
SELECT Add(3, 4);

See discussion here: googleapis/python-bigquery#377

This PR solves this problem by running queries as a single statement

BEFORE/AFTER SCREENSHOTS OR ANIMATED GIF

TEST PLAN

ADDITIONAL INFORMATION

• Has associated issue:
• Changes UI
• Requires DB Migration.
• Confirm DB Migration upgrade and downgrade tested.
• Introduces new feature or API
• Removes existing feature or API

[codecov-io]

Codecov Report

Merging #11904 (7144adc) into master (721f911) will decrease coverage by 0.00%.
The diff coverage is 77.77%.

@@            Coverage Diff             @@
##           master   #11904      +/-   ##
==========================================
- Coverage   63.78%   63.78%   -0.01%     
==========================================
  Files         927      927              
  Lines       45006    45011       +5     
  Branches     4308     4308              
==========================================
+ Hits        28708    28711       +3     
- Misses      16120    16122       +2     
  Partials      178      178              

Flag	Coverage Δ
javascript	63.17% <ø> (ø)
python	64.14% <77.77%> (-0.01%)	⬇️

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Impacted Files	Coverage Δ
superset/sql_lab.py	79.62% <71.42%> (-0.67%)	⬇️
superset/db_engine_specs/base.py	87.50% <100.00%> (+0.03%)	⬆️
superset/db_engine_specs/bigquery.py	95.74% <100.00%> (+0.04%)	⬆️

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[villebro]

Nice, super happy to see this collaborative effort materialize like this! Minor non-blocking nit, LGTM.