[BUG] DLQ Serialization Doesn't Support Create Action

[engechas]

Describe the bug
DataPrepper supports create and index actions in the OpenSearch sink. Depending on which action is specified, the BulkRequest populates the respective parameter. This is accounted for in some places of the code, such as the JavaClientUncompressedAccumulatingBulkRequest, but is not accounted for in the DLQ serialization logic:
https://github.com/opensearch-project/data-prepper/blob/main/data-prepper-plugins/opensearch/src/main/java/org/opensearch/dataprepper/plugins/sink/opensearch/dlq/FailedBulkOperationConverter.java#L65

This causes an exception that shuts down DataPrepper:

ERROR org.opensearch.dataprepper.pipeline.common.FutureHelper - FutureTask failed due to: 
java.util.concurrent.ExecutionException: java.lang.IllegalStateException: Cannot get 'Index' variant: current variant is 'Create'.
    at java.util.concurrent.FutureTask.report(FutureTask.java:122) ~[?:?]
    at java.util.concurrent.FutureTask.get(FutureTask.java:191) ~[?:?]
    at org.opensearch.dataprepper.pipeline.common.FutureHelper.awaitFuturesIndefinitely(FutureHelper.java:29) ~[data-prepper-core-2.3.1.jar:?]
    at org.opensearch.dataprepper.pipeline.ProcessWorker.postToSink(ProcessWorker.java:140) ~[data-prepper-core-2.3.1.jar:?]
    at org.opensearch.dataprepper.pipeline.ProcessWorker.doRun(ProcessWorker.java:121) ~[data-prepper-core-2.3.1.jar:?]
    at org.opensearch.dataprepper.pipeline.ProcessWorker.run(ProcessWorker.java:50) ~[data-prepper-core-2.3.1.jar:?]
    at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:515) ~[?:?]
    at java.util.concurrent.FutureTask.run(FutureTask.java:264) ~[?:?]
    at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1128) ~[?:?]
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:628) ~[?:?]
    at java.lang.Thread.run(Thread.java:829) [?:?]
Caused by: java.lang.IllegalStateException: Cannot get 'Index' variant: current variant is 'Create'.
    at org.opensearch.client.util.TaggedUnionUtils.get(TaggedUnionUtils.java:47) ~[opensearch-java-2.5.0.jar:?]
    at org.opensearch.client.opensearch.core.bulk.BulkOperation.index(BulkOperation.java:139) ~[opensearch-java-2.5.0.jar:?]
    at org.opensearch.dataprepper.plugins.sink.opensearch.dlq.FailedBulkOperationConverter.convertDocumentToGenericMap(FailedBulkOperationConverter.java:65) ~[opensearch-2.3.1.jar:?]
    at org.opensearch.dataprepper.plugins.sink.opensearch.dlq.FailedBulkOperationConverter.convertToDlqObject(FailedBulkOperationConverter.java:40) ~[opensearch-2.3.1.jar:?]
    at java.util.stream.ReferencePipeline$3$1.accept(ReferencePipeline.java:195) ~[?:?]
    at java.util.Spliterators$ArraySpliterator.forEachRemaining(Spliterators.java:948) ~[?:?]
    at java.util.stream.AbstractPipeline.copyInto(AbstractPipeline.java:484) ~[?:?]
    at java.util.stream.AbstractPipeline.wrapAndCopyInto(AbstractPipeline.java:474) ~[?:?]
    at java.util.stream.ReduceOps$ReduceOp.evaluateSequential(ReduceOps.java:913) ~[?:?]
    at java.util.stream.AbstractPipeline.evaluate(AbstractPipeline.java:234) ~[?:?]
    at java.util.stream.ReferencePipeline.collect(ReferencePipeline.java:578) ~[?:?]
    at org.opensearch.dataprepper.plugins.sink.opensearch.OpenSearchSink.logFailureForBulkRequests(OpenSearchSink.java:328) ~[opensearch-2.3.1.jar:?]
    at org.opensearch.dataprepper.plugins.sink.opensearch.BulkRetryStrategy.handleFailures(BulkRetryStrategy.java:334) ~[opensearch-2.3.1.jar:?]
    at org.opensearch.dataprepper.plugins.sink.opensearch.BulkRetryStrategy.handleFailures(BulkRetryStrategy.java:252) ~[opensearch-2.3.1.jar:?]
    at org.opensearch.dataprepper.plugins.sink.opensearch.BulkRetryStrategy.handleRetriesAndFailures(BulkRetryStrategy.java:245) ~[opensearch-2.3.1.jar:?]
    at org.opensearch.dataprepper.plugins.sink.opensearch.BulkRetryStrategy.handleRetry(BulkRetryStrategy.java:269) ~[opensearch-2.3.1.jar:?]
    at org.opensearch.dataprepper.plugins.sink.opensearch.BulkRetryStrategy.execute(BulkRetryStrategy.java:191) ~[opensearch-2.3.1.jar:?]
    at org.opensearch.dataprepper.plugins.sink.opensearch.OpenSearchSink.lambda$flushBatch$6(OpenSearchSink.java:314) ~[opensearch-2.3.1.jar:?]
    at io.micrometer.core.instrument.composite.CompositeTimer.record(CompositeTimer.java:141) ~[micrometer-core-1.10.5.jar:1.10.5]
    at org.opensearch.dataprepper.plugins.sink.opensearch.OpenSearchSink.flushBatch(OpenSearchSink.java:311) ~[opensearch-2.3.1.jar:?]
    at org.opensearch.dataprepper.plugins.sink.opensearch.OpenSearchSink.doOutput(OpenSearchSink.java:283) ~[opensearch-2.3.1.jar:?]
    at org.opensearch.dataprepper.model.sink.AbstractSink.lambda$output$0(AbstractSink.java:64) ~[data-prepper-api-2.3.1.jar:?]
    at io.micrometer.core.instrument.composite.CompositeTimer.record(CompositeTimer.java:141) ~[micrometer-core-1.10.5.jar:1.10.5]
    at org.opensearch.dataprepper.model.sink.AbstractSink.output(AbstractSink.java:64) ~[data-prepper-api-2.3.1.jar:?]
    at org.opensearch.dataprepper.pipeline.Pipeline.lambda$publishToSinks$5(Pipeline.java:336) ~[data-prepper-core-2.3.1.jar:?]

There may be other areas where the assumption that action is index is made. The code should be audited to ensure all document accesses account for both supported actions.

Expected behavior
DLQ serialization works regardless of action