[BUG] Support custom index type for private opensearch serverless collection as sink

[oeyh]

Is your feature request related to a problem? Please describe.
As a Data Prepper user, I'd like to use custom index type in opensearch sink to set up index template when using vpc collection as sink.

Current issue with this setup is when opensearch sink initializes, it tries to set up index and index template before creating or updating serverless network policy, which results in 401 errors.

Trackback looks like this:

2024-03-07T21:04:55,273 [log-pipeline-sink-worker-2-thread-1] INFO  org.opensearch.dataprepper.plugins.sink.opensearch.index.AbstractIndexManager - Index template vpc-aoss-test-000003-index-template does not exist and should be created
2024-03-07T21:04:55,285 [log-pipeline-sink-worker-2-thread-1] WARN  org.opensearch.dataprepper.plugins.sink.opensearch.OpenSearchSink - Failed to initialize OpenSearch sink with a retryable exception.
org.opensearch.client.opensearch._types.OpenSearchException: Request failed: [http_exception] server returned 401
	at org.opensearch.client.transport.aws.AwsSdk2Transport.parseResponse(AwsSdk2Transport.java:494) ~[opensearch-java-2.8.1.jar:?]
	at org.opensearch.client.transport.aws.AwsSdk2Transport.executeSync(AwsSdk2Transport.java:392) ~[opensearch-java-2.8.1.jar:?]
	at org.opensearch.client.transport.aws.AwsSdk2Transport.performRequest(AwsSdk2Transport.java:192) ~[opensearch-java-2.8.1.jar:?]
	at org.opensearch.dataprepper.plugins.sink.opensearch.index.ComposableTemplateAPIWrapper.putTemplate(ComposableTemplateAPIWrapper.java:34) ~[opensearch-2.7.0-SNAPSHOT.jar:?]
	at org.opensearch.dataprepper.plugins.sink.opensearch.index.ComposableIndexTemplateStrategy.createTemplate(ComposableIndexTemplateStrategy.java:47) ~[opensearch-2.7.0-SNAPSHOT.jar:?]
	at org.opensearch.dataprepper.plugins.sink.opensearch.index.AbstractIndexManager.checkAndCreateIndexTemplate(AbstractIndexManager.java:258) ~[opensearch-2.7.0-SNAPSHOT.jar:?]
	at org.opensearch.dataprepper.plugins.sink.opensearch.index.AbstractIndexManager.checkAndCreateIndexTemplate(AbstractIndexManager.java:234) ~[opensearch-2.7.0-SNAPSHOT.jar:?]
	at org.opensearch.dataprepper.plugins.sink.opensearch.index.AbstractIndexManager.setupIndex(AbstractIndexManager.java:224) ~[opensearch-2.7.0-SNAPSHOT.jar:?]
	at org.opensearch.dataprepper.plugins.sink.opensearch.OpenSearchSink.doInitializeInternal(OpenSearchSink.java:231) ~[opensearch-2.7.0-SNAPSHOT.jar:?]
	at org.opensearch.dataprepper.plugins.sink.opensearch.OpenSearchSink.doInitialize(OpenSearchSink.java:193) ~[opensearch-2.7.0-SNAPSHOT.jar:?]
	at org.opensearch.dataprepper.model.sink.AbstractSink.initialize(AbstractSink.java:52) ~[data-prepper-api-2.7.0-SNAPSHOT.jar:?]
	at org.opensearch.dataprepper.pipeline.Pipeline.isReady(Pipeline.java:200) ~[data-prepper-core-2.7.0-SNAPSHOT.jar:?]
	at org.opensearch.dataprepper.pipeline.Pipeline.lambda$execute$2(Pipeline.java:252) ~[data-prepper-core-2.7.0-SNAPSHOT.jar:?]

Describe the solution you'd like
Support custom index type for private opensearch serverless collection as sink. This can probably be resolved by just moving network policy update code ahead of setting up index.

Describe alternatives you've considered (Optional)
N/A

Additional context
N/A