Fix issues with using BigQuery Connection created from UI

[kaxil]

This PR creates a constant for CONN_TYPE_TO_DATABASE and adds gcpbigquery to it. It then uses this for other places .

closes #252

[codecov]

Codecov Report

Merging #294 (7cbaa91) into main (1ab6f7a) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #294   +/-   ##
=======================================
  Coverage   92.28%   92.28%           
=======================================
  Files          35       35           
  Lines        1530     1530           
  Branches      273      273           
=======================================
  Hits         1412     1412           
  Misses         86       86           
  Partials       32       32           

Impacted Files	Coverage Δ
src/astro/sql/operators/agnostic_stats_check.py	94.73% <ø> (ø)
src/astro/utils/__init__.py	82.35% <ø> (ø)
src/astro/constants.py	100.00% <100.00%> (ø)
src/astro/utils/database.py	100.00% <100.00%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 1ab6f7a...7cbaa91. Read the comment docs.

[kaxil]

Would love reviews on this @utkarsharma2 @tatiana