Only keep the first element of multivalue field response.

[penghuo]

Issue #, if available: #1017

Description of changes:

1. Only keep the first element of multivalue field response. https://github.com/penghuo/sql/blob/issue-1017/docs/experiment/ppl/general/datatypes.rst#example-3-selecting-field-of-array-value

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 #1026 (3276e77) into develop (1dbf244) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@              Coverage Diff              @@
##             develop    #1026      +/-   ##
=============================================
- Coverage      99.87%   99.87%   -0.01%     
- Complexity      2416     2431      +15     
=============================================
  Files            234      236       +2     
  Lines           5547     5535      -12     
  Branches         359      332      -27     
=============================================
- Hits            5540     5528      -12     
  Misses             5        5              
  Partials           2        2              

Impacted Files	Coverage Δ	Complexity Δ
...ticsearch/data/utils/ElasticsearchJsonContent.java	100.00% <100.00%> (ø)	20.00 <20.00> (?)
...ch/sql/elasticsearch/data/utils/ObjectContent.java	100.00% <100.00%> (ø)	19.00 <19.00> (?)
...arch/data/value/ElasticsearchExprValueFactory.java	100.00% <100.00%> (ø)	36.00 <30.00> (-24.00)

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[dai-chen]

Thanks for the changes!