Add _routing to SQL includes list (#277)

[acarbonetto]

Description

Add the _routing reserved word as a metafield accessible as an identifier from any opensearch index.

Example:

select _id, _index, _routing, int0 from calcs limit 5

Example response:

        [
            "1",
            "calcs",
            "[xoyIAOdqQsS74x2yKSgiEQ][calcs][0]",
            1
        ],

Issues Resolved

Partially resolves: sql#1478

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]

Codecov Report

Merging #1762 (aca7694) into main (c7dfdb3) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##               main    #1762   +/-   ##
=========================================
  Coverage     97.29%   97.29%           
- Complexity     4408     4409    +1     
=========================================
  Files           388      388           
  Lines         10944    10946    +2     
  Branches        774      775    +1     
=========================================
+ Hits          10648    10650    +2     
  Misses          289      289           
  Partials          7        7           

Flag	Coverage Δ
sql-engine	97.29% <100.00%> (+<0.01%)	⬆️

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Impacted Files	Coverage Δ
...search/sql/opensearch/storage/OpenSearchIndex.java	100.00% <ø> (ø)
...ch/sql/opensearch/response/OpenSearchResponse.java	100.00% <100.00%> (ø)

[Yury-Fridlyand]

Can this be used in WHERE? Can you update docs (doctest) as well?