Introduce mapping transformer to allow transform mappings during index create/update or index template create/update

[bzhangam]

Description

In this PR we introduce the mapping transformer to the core to allow transforming the mapping during index create/update and index template create/update. We want to do this because we have a use case in neural search plugin where we want to auto generate the semantic fields(e.g. knn_vector or rank_feature) in the mappings based on the model id defined in the semantic field(A new field type introduced by neural search plugin). In this way we can simplify the neural search set up.

Potentially in future this feature can be leveraged by other plugins to auto transform the mapping based on the certain config defined by users.

Related Issues

Resolves #[17500] - Proposal to introduce the mapping transformer into core.

#[803] - Proposal to support semantic field in neural search.
#[1211] - HLD of the semantic field.

Check List

• Functionality includes testing.
• API changes companion pull request created, if applicable.
• Public documentation issue/PR created, if applicable.

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[github-actions]

❕ Gradle check result for ceabd64: UNSTABLE

Please review all flaky tests that succeeded after retry and create an issue if one does not already exist to track the flaky failure.

[codecov]

Codecov Report

Attention: Patch coverage is 83.59375% with 21 lines in your changes missing coverage. Please review.

Project coverage is 72.53%. Comparing base (5799fe7) to head (c57fbf4).
Report is 6 commits behind head on main.

Files with missing lines	Patch %	Lines
...earch/index/mapper/MappingTransformerRegistry.java	80.48%	8 Missing ⚠️
...put/TransportPutComposableIndexTemplateAction.java	82.35%	1 Missing and 2 partials ⚠️
.../template/put/TransportPutIndexTemplateAction.java	85.00%	3 Missing ⚠️
...rg/opensearch/index/mapper/MappingTransformer.java	0.00%	3 Missing ⚠️
...plate/put/TransportPutComponentTemplateAction.java	87.50%	0 Missing and 2 partials ⚠️
...java/org/opensearch/cluster/metadata/Template.java	0.00%	2 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               main   #17635      +/-   ##
============================================
- Coverage     72.57%   72.53%   -0.05%     
- Complexity    67066    67103      +37     
============================================
  Files          5470     5475       +5     
  Lines        309697   309903     +206     
  Branches      45045    45065      +20     
============================================
+ Hits         224776   224790      +14     
- Misses        66602    66711     +109     
- Partials      18319    18402      +83     

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[github-actions]

❕ Gradle check result for 077b13c: UNSTABLE

Please review all flaky tests that succeeded after retry and create an issue if one does not already exist to track the flaky failure.

[github-actions]

❌ Gradle check result for c88378f: FAILURE

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[github-actions]

❌ Gradle check result for c88378f: FAILURE

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[github-actions]

❌ Gradle check result for 70be6d0: FAILURE

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[github-actions]

❕ Gradle check result for c57fbf4: UNSTABLE

Please review all flaky tests that succeeded after retry and create an issue if one does not already exist to track the flaky failure.

[saratvemulapalli]

Thanks for taking care of the feedback @bzhangam !