feat: add support for JOIN in Druid

[betodealmeida]

SUMMARY

Druid has supported JOIN for a quite a while. I added a feature flag to turn it on, and hopefully in 2.0 we can make it on by default.

BEFORE/AFTER SCREENSHOTS OR ANIMATED GIF

With the flag off we get 2 queries (1 to get top groups, the other filtering on top groups) in a line chart:

With the flag on we get a join:

TESTING INSTRUCTIONS

ADDITIONAL INFORMATION

• Has associated issue:
• Required feature flags:
• Changes UI
• Includes DB Migration (follow approval process in SIP-59)
  • Migration is atomic, supports rollback & is backwards-compatible
  • Confirm DB migration upgrade and downgrade tested
  • Runtime estimates and downtime expectations provided
• Introduces new feature or API
• Removes existing feature or API

[codecov]

Codecov Report

Merging #16770 (5a3dccb) into master (d375538) will decrease coverage by 0.20%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master   #16770      +/-   ##
==========================================
- Coverage   76.96%   76.76%   -0.21%     
==========================================
  Files        1007     1007              
  Lines       54182    54184       +2     
  Branches     7464     7464              
==========================================
- Hits        41700    41593     -107     
- Misses      12242    12351     +109     
  Partials      240      240              

Flag	Coverage Δ
hive	?
mysql	81.70% <100.00%> (?)
postgres	81.80% <100.00%> (-0.01%)	⬇️
presto	?
python	81.88% <100.00%> (-0.39%)	⬇️
sqlite	81.41% <100.00%> (+0.03%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
superset/config.py	91.44% <ø> (ø)
superset/db_engine_specs/druid.py	88.88% <100.00%> (+0.25%)	⬆️
superset/db_engines/hive.py	0.00% <0.00%> (-82.15%)	⬇️
superset/db_engine_specs/hive.py	69.80% <0.00%> (-16.87%)	⬇️
superset/db_engine_specs/presto.py	83.47% <0.00%> (-6.91%)	⬇️
superset/views/database/mixins.py	81.03% <0.00%> (-1.73%)	⬇️
superset/connectors/sqla/models.py	86.31% <0.00%> (-1.64%)	⬇️
superset/datasets/commands/importers/v1/utils.py	59.25% <0.00%> (-0.75%)	⬇️
superset/db_engine_specs/base.py	88.20% <0.00%> (-0.39%)	⬇️
superset/utils/core.py	89.84% <0.00%> (-0.13%)	⬇️
... and 3 more

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