feat(sqla): add time grain and time column to jinja params

[villebro]

SUMMARY

Add time_grain and time_column variables to the default Jinja context. Also add default Jinja variables to the documentation.

With the following virtual dataset:

The generated query looks as follows:

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 #16680 (1de4d66) into master (51acada) will decrease coverage by 0.11%.
The diff coverage is 94.52%.

❗ Current head 1de4d66 differs from pull request most recent head 0214e97. Consider uploading reports for the commit 0214e97 to get more accurate results

@@            Coverage Diff             @@
##           master   #16680      +/-   ##
==========================================
- Coverage   76.91%   76.80%   -0.12%     
==========================================
  Files        1005     1005              
  Lines       54017    54049      +32     
  Branches     7337     7339       +2     
==========================================
- Hits        41549    41512      -37     
- Misses      12228    12297      +69     
  Partials      240      240              

Flag	Coverage Δ
hive	?
mysql	81.65% <95.23%> (+0.02%)	⬆️
postgres	81.67% <95.31%> (-0.02%)	⬇️
presto	81.52% <95.31%> (+0.02%)	⬆️
python	81.98% <95.31%> (-0.23%)	⬇️
sqlite	81.28% <95.23%> (-0.02%)	⬇️

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

Impacted Files	Coverage Δ
...src/dashboard/components/gridComponents/Column.jsx	87.87% <ø> (ø)
...nd/src/dashboard/components/gridComponents/Row.jsx	86.66% <ø> (ø)
...perset-frontend/src/dashboard/util/isValidChild.ts	85.71% <ø> (-0.96%)	⬇️
superset/connectors/sqla/models.py	89.46% <75.00%> (-0.24%)	⬇️
...ashboard/components/gridComponents/ChartHolder.jsx	69.81% <87.50%> (+0.50%)	⬆️
superset/views/core.py	76.63% <96.15%> (+0.38%)	⬆️
...nd/src/dashboard/containers/DashboardComponent.jsx	100.00% <100.00%> (ø)
superset/utils/sqllab_execution_context.py	92.37% <100.00%> (+0.55%)	⬆️
superset/db_engines/hive.py	0.00% <0.00%> (-82.15%)	⬇️
superset/db_engine_specs/hive.py	69.80% <0.00%> (-16.87%)	⬇️
... and 2 more

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[zhaoyongjie]

nice!

[zhaoyongjie]

LGTM.