[Backport 2.x] Add template snippets support for field and target_field in KV ingest processor

[gaobinlong]

Description

Backport #10040.

Related Issues

#9964

Check List

• New functionality includes testing.
  • All tests pass
• New functionality has been documented.
  • New functionality has javadoc added
• Failing checks are inspected and point to the corresponding known issue(s) (See: Troubleshooting Failing Builds)
• Commits are signed per the DCO using --signoff
• Commit changes are listed out in CHANGELOG.md file (See: Changelog)
• Public documentation issue/PR created

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[github-actions]

Compatibility status:

Checks if related components are compatible with change 591a190

Incompatible components

Incompatible components: [https://github.com/opensearch-project/performance-analyzer.git]

Skipped components

Compatible components

Compatible components: [https://github.com/opensearch-project/security-analytics.git, https://github.com/opensearch-project/asynchronous-search.git, https://github.com/opensearch-project/observability.git, https://github.com/opensearch-project/notifications.git, https://github.com/opensearch-project/reporting.git, https://github.com/opensearch-project/custom-codecs.git, https://github.com/opensearch-project/job-scheduler.git, https://github.com/opensearch-project/neural-search.git, https://github.com/opensearch-project/common-utils.git, https://github.com/opensearch-project/geospatial.git, https://github.com/opensearch-project/anomaly-detection.git, https://github.com/opensearch-project/ml-commons.git, https://github.com/opensearch-project/index-management.git, https://github.com/opensearch-project/security.git, https://github.com/opensearch-project/cross-cluster-replication.git, https://github.com/opensearch-project/alerting.git, https://github.com/opensearch-project/sql.git, https://github.com/opensearch-project/k-nn.git, https://github.com/opensearch-project/performance-analyzer-rca.git]

[github-actions]

❕ Gradle check result for 591a190: UNSTABLE

• TEST FAILURES:

      1 org.opensearch.search.SearchWeightedRoutingIT.testMultiGetWithNetworkDisruption_FailOpenEnabled

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: 4 lines in your changes are missing coverage. Please review.

Comparison is base (847cf9c) 71.02% compared to head (591a190) 71.07%.

Files	Patch %	Lines
...rg/opensearch/ingest/common/KeyValueProcessor.java	91.66%	2 Missing and 1 partial ⚠️
...g/opensearch/ingest/common/IngestCommonPlugin.java	0.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##                2.x   #11300      +/-   ##
============================================
+ Coverage     71.02%   71.07%   +0.05%     
- Complexity    59071    59138      +67     
============================================
  Files          4869     4869              
  Lines        278636   278654      +18     
  Branches      40826    40830       +4     
============================================
+ Hits         197888   198058     +170     
+ Misses        63996    63855     -141     
+ Partials      16752    16741      -11     

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