[Enhancement] Support custom index template for ES 6 in opensearch sink

Is your feature request related to a problem? Please describe.
Currently the index_type defaults to management_disabled when distribution_version is set to es6. This is due to old index mapping schema in ES 6:

org.opensearch.client.opensearch._types.OpenSearchException: Request failed: [illegal_argument_exception] Malformed [mappings] section for type [dynamic_templates], should include an inner object describing the mapping
	at org.opensearch.client.transport.aws.AwsSdk2Transport.parseResponse(AwsSdk2Transport.java:492) ~[opensearch-java-2.5.0.jar:?]

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Start by tracing how the OpenSearch sink handles index_type and distribution_version for ES 6, using the reported malformed mappings error as the initial symptom. The issue does not name files, tests, or a precise template configuration; completion would require defining custom index-template behavior and verifying that ES 6 mappings are accepted.