Extend the PPL identifier defintion

[penghuo]

Issue #, if available: #882

Description of changes:

1. Update the doc to support Elasticsearch extension as identifier. https://github.com/penghuo/sql/blob/issue-882/docs/experiment/ppl/general/identifiers.rst

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 #888 (d7bbb3c) into develop (f47e9fc) will increase coverage by 0.00%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             develop     #888   +/-   ##
==========================================
  Coverage      99.85%   99.85%           
- Complexity      2132     2145   +13     
==========================================
  Files            215      216    +1     
  Lines           4774     4832   +58     
  Branches         313      318    +5     
==========================================
+ Hits            4767     4825   +58     
  Misses             5        5           
  Partials           2        2           

Impacted Files	Coverage Δ	Complexity Δ
...ch/planner/logical/rule/OptimizationRuleUtils.java	100.00% <0.00%> (ø)	6.00% <0.00%> (+4.00%)
.../planner/logical/ElasticsearchLogicalIndexAgg.java	100.00% <0.00%> (ø)	2.00% <0.00%> (ø%)
...ge/script/aggregation/AggregationQueryBuilder.java	100.00% <0.00%> (ø)	7.00% <0.00%> (+1.00%)
...ript/aggregation/dsl/BucketAggregationBuilder.java	100.00% <0.00%> (ø)	3.00% <0.00%> (ø%)
...ical/ElasticsearchLogicalPlanOptimizerFactory.java	100.00% <0.00%> (ø)	1.00% <0.00%> (ø%)
...rch/planner/logical/rule/MergeSortAndIndexAgg.java	100.00% <0.00%> (ø)	8.00% <0.00%> (?%)

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[dai-chen]

LGTM. Thanks!