Refine PPL head command syntax

[chloe-zh]

Issue #, if available:
#930

Description of changes:

• Modified head command syntax to simply head [N] where the optional argument N is the number of results to return
• Removed head operator and replaced the original use of head operator with limit operator
• Updated user manual

Breaking Changes Note: We have made breaking changes in the syntax of head command from previous head [keeplast = (true | false)] [while "("<boolean-expression>")"] [N] to current head [N]. We are no longer supporting the keeplast and while options.

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 #1022 (1340e91) into develop (614c500) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@              Coverage Diff              @@
##             develop    #1022      +/-   ##
=============================================
+ Coverage      99.87%   99.89%   +0.01%     
+ Complexity      2409     2402       -7     
=============================================
  Files            234      234              
  Lines           5527     5464      -63     
  Branches         358      326      -32     
=============================================
- Hits            5520     5458      -62     
  Misses             5        5              
+ Partials           2        1       -1     

Impacted Files	Coverage Δ	Complexity Δ
...endistroforelasticsearch/sql/executor/Explain.java	100.00% <ø> (ø)	27.00 <0.00> (-1.00)
...relasticsearch/sql/planner/DefaultImplementor.java	100.00% <ø> (ø)	15.00 <0.00> (-1.00)
...sticsearch/sql/planner/logical/LogicalPlanDSL.java	100.00% <ø> (ø)	15.00 <0.00> (-1.00)
...ch/sql/planner/logical/LogicalPlanNodeVisitor.java	100.00% <ø> (ø)	15.00 <0.00> (-1.00)
...icsearch/sql/planner/physical/PhysicalPlanDSL.java	100.00% <ø> (ø)	14.00 <0.00> (-1.00)
.../sql/planner/physical/PhysicalPlanNodeVisitor.java	100.00% <ø> (ø)	15.00 <0.00> (-1.00)
...tor/protector/ElasticsearchExecutionProtector.java	100.00% <ø> (ø)	19.00 <0.00> (-1.00)
...orelasticsearch/sql/ppl/utils/ArgumentFactory.java	100.00% <ø> (+1.61%)	24.00 <0.00> (-4.00)	⬆️
...ndistroforelasticsearch/sql/analysis/Analyzer.java	100.00% <100.00%> (ø)	43.00 <0.00> (-1.00)
...troforelasticsearch/sql/ppl/parser/AstBuilder.java	100.00% <100.00%> (ø)	33.00 <2.00> (ø)
... and 15 more

---

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 614c500...1340e91. Read the comment docs.

[dai-chen]

Just a minor comment. Thanks for the refactoring!

[penghuo]

Thanks for the change!