Set target routing shard by partition key (#316)

[acarbonetto]

Description

Allows user to set routing shard via the relation partition keys. For example:

select _id, _index, _routing, int0 from calcs_routing PARTITION(KEY1, KEY2) where _routing = 'KEY1' or _routing = 'KEY2'

Note that multiple keys can be send to OpenSearch storage by using comma-delimited list.

IT tests

New IT tests running against a multi-node cluster have been added to the build. These can be run using ./gradlew :integ-test:multiClusterSearch and will be automatically run against CI.

TODO

• [ ] Rebase
• [ ] Update IT test strategy
• [ ] Propose PPL syntax

Issues Resolved

• fixes https://github.com/opensearch-project/sql/issues/1478 (SQL)

Note (out of scope): PPL syntax is still TBD

Check List

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

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