[Backport] implement transport api for PPL inter-plugin communication (#533)

[penghuo]

Signed-off-by: Zhongnan Su [email protected]

Check List

• New functionality includes testing.
  • All tests pass, including unit test, integration test and doctest
• New functionality has been documented.
  • New functionality has javadoc added
  • New functionality has user manual doc added
• Commits are signed per the DCO using --signoff

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[codecov-commenter]

Codecov Report

Merging #778 (176d038) into 2.1 (0576d96) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##                2.1     #778   +/-   ##
=========================================
  Coverage     97.71%   97.71%           
  Complexity     2801     2801           
=========================================
  Files           271      271           
  Lines          6908     6908           
  Branches        438      438           
=========================================
  Hits           6750     6750           
  Misses          157      157           
  Partials          1        1           

Flag	Coverage Δ
sql-engine	97.71% <100.00%> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
...org/opensearch/sql/ppl/domain/PPLQueryRequest.java	100.00% <100.00%> (ø)

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