Support SELECT DISTINCT in new SQL engine

[dai-chen]

Issue #, if available: #582

Description of changes: Change SQL grammar and build AST for DISTINCT keyword. Currently distinct is treated as an equivalent and special form of GROUP BY clause. So there is no change required in core engine.

TODOs:

1. Investigate how to support DISTINCT * in Elasticsearch (add note in doc)
2. Migrate aggregate function with distinct to new engine as well, ex. COUNT(DISTINCT field).
3. For the bug fix, will open a separate PR since I found CASE statement has problem with pushdown optimization.

Documentation: Add distinct * limitation and new support for distinct expression in https://github.com/dai-chen/sql/blob/support-distinct-in-new-engine/docs/user/dql/basics.rst#example-4-selecting-distinct-fields

Testing: Add UT and comparison test.

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 #833 (1b8a2f9) into develop (9f65c67) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             develop     #833   +/-   ##
==========================================
  Coverage      99.85%   99.85%           
  Complexity      2148     2148           
==========================================
  Files            216      216           
  Lines           4845     4850    +5     
  Branches         318      320    +2     
==========================================
+ Hits            4838     4843    +5     
  Misses             5        5           
  Partials           2        2           

Impacted Files	Coverage Δ	Complexity Δ
...icsearch/sql/sql/parser/AstAggregationBuilder.java	100.00% <100.00%> (ø)	15.00 <3.00> (ø)
...rch/sql/sql/parser/context/QuerySpecification.java	100.00% <100.00%> (ø)	15.00 <0.00> (ø)

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