Support NULLS FIRST/LAST in new engine

[dai-chen]

Issue #, if available: N/A

Description of changes:

1. Change SQL grammar to add NULLS FIRST and NULLS LAST clause.
2. Build AST Sort node with sort options given.
3. Analyze sort options by determining default nulls order if NULLS clause absent.
4. Change comparison test data and framework to support nulls.

TODO: Add support in window function OVER clause. Because AST window function is unresolved expression instead of operator, regular ORDER BY and that in over clause cannot share same logic.

Documentation:

1. Add doctest: https://github.com/dai-chen/sql/blob/support-order-by-null-order-2/docs/user/dql/basics.rst#example-2-specifying-order-for-null
2. Update README with all features only available in new engine to avoid user confusion: https://github.com/dai-chen/sql/tree/support-order-by-null-order-2#experimental

Testing:

1. Add UT and comparison test
2. Remove one comparison test query because it is ordering on SUM(FlightDelayMin) which is 0 for many rows and the order is not determined.
3. Change alias names in can_build_from_subquery() because new SQL doesn't support keyword as alias name.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[codecov]

Codecov Report

Merging #843 (173911f) into develop (4ebe901) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             develop     #843   +/-   ##
==========================================
  Coverage      99.84%   99.85%           
- Complexity      2094     2102    +8     
==========================================
  Files            209      209           
  Lines           4658     4678   +20     
  Branches         301      308    +7     
==========================================
+ Hits            4651     4671   +20     
  Misses             5        5           
  Partials           2        2           

Impacted Files	Coverage Δ	Complexity Δ
...ndistroforelasticsearch/sql/analysis/Analyzer.java	100.00% <100.00%> (ø)	43.00 <6.00> (+5.00)
...orelasticsearch/sql/sql/parser/AstSortBuilder.java	100.00% <100.00%> (ø)	7.00 <4.00> (+3.00)
...rch/sql/sql/parser/context/QuerySpecification.java	100.00% <100.00%> (ø)	15.00 <0.00> (ø)

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[penghuo]

Thanks for the change.