[Pull-based Ingestion] Support message mappers to support different input formats and raw payloads

[varunbharadwaj]

Description

This PR refactors the pull-based indexing flow to support message mappers. A default message mapper is created to retain current behavior. Alternatively, a raw payload mapper is added to support ingesting from any given streaming source.

In the raw payload mode, the Kafka offset / Kinesis sequence number will be used as the document ID. This will ensure duplicate documents are not created on rewind/replay. Document versioning will not be supported, and only an eventually consistent view of documents can be expected on message replays (as older message can potentially overwrite newer one on replay, until the lag is caught up). This will be an append-only indexing mode.

This model should allow the flexibility to support other formats in the future, when needed.

Related Issues

Resolves #19548

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❌ Gradle check result for 72773f3: FAILURE

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✅ Gradle check result for 85dae2e: SUCCESS

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❌ Gradle check result for e9e9f3f: FAILURE

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✅ Gradle check result for e9e9f3f: SUCCESS

[codecov]

Codecov Report

❌ Patch coverage is 92.72727% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 73.10%. Comparing base (753c135) to head (e9e9f3f).
⚠️ Report is 4 commits behind head on main.

Files with missing lines	Patch %	Lines
...g/opensearch/cluster/metadata/IngestionSource.java	75.00%	0 Missing and 2 partials ⚠️
.../pollingingest/mappers/IngestionMessageMapper.java	88.23%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               main   #19765      +/-   ##
============================================
- Coverage     73.10%   73.10%   -0.01%     
+ Complexity    70959    70932      -27     
============================================
  Files          5737     5740       +3     
  Lines        324766   324816      +50     
  Branches      46981    46986       +5     
============================================
+ Hits         237425   237460      +35     
- Misses        68226    68245      +19     
+ Partials      19115    19111       -4     

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