Support NULLS FIRST/LAST ordering for window functions

[dai-chen]

Issue #, if available: #928

Description of changes: Previously window function only parses sorting order ASC/DESC. This PR is to support null ordering NULLS FIRST/LAST for window function OVER clause too.

The basic logic is same as ORDER BY clause handling:

1. SQL frontend just pass sorting and null ordering given in queries.
2. The final ordering is determined in analyzer which has the knowledge of default ordering option.

Testing: Because both H2 and SQLite has problem with null ordering in OVER clause, a new IT class is added manually.

Documentation: https://github.com/dai-chen/sql/blob/support-null-first-last-for-window-function/docs/user/dql/window.rst#row-number

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 #929 (5b2b9a7) into develop (0855c53) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             develop     #929   +/-   ##
==========================================
  Coverage      99.87%   99.87%           
- Complexity      2341     2347    +6     
==========================================
  Files            232      232           
  Lines           5389     5395    +6     
  Branches         350      350           
==========================================
+ Hits            5382     5388    +6     
  Misses             5        5           
  Partials           2        2           

Impacted Files	Coverage Δ	Complexity Δ
...csearch/sql/analysis/WindowExpressionAnalyzer.java	100.00% <100.00%> (ø)	12.00 <4.00> (+2.00)
...ticsearch/sql/sql/parser/AstExpressionBuilder.java	100.00% <100.00%> (ø)	48.00 <1.00> (-2.00)
...roforelasticsearch/sql/sql/parser/ParserUtils.java	100.00% <100.00%> (ø)	7.00 <6.00> (+6.00)
...rch/sql/sql/parser/context/QuerySpecification.java	100.00% <100.00%> (ø)	15.00 <0.00> (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 0855c53...5b2b9a7. Read the comment docs.

[penghuo]

LGTM.