Move submission of exectuor service to end of opensearch sink initialization

[graytaylor0]

Description

If using the query_lookup and there is an exception initializing the sink, retries will result in a RejectedExecutionException.

WARN  org.opensearch.dataprepper.plugins.sink.opensearch.OpenSearchSink - Failed to initialize OpenSearch sink with a retryable exception. 
java.util.concurrent.RejectedExecutionException: Task java.util.concurrent.FutureTask@7341b399[Not completed, task = java.util.concurrent.Executors$RunnableAdapter@49b256e5[Wrapped task = org.opensearch.dataprepper.plugins.sink.opensearch.index.ExistingDocumentQueryManager@11827a57]] rejected from java.util.concurrent.ThreadPoolExecutor@344ae185[Terminated, pool size = 0, active threads = 0, queued tasks = 0, completed tasks = 1]
	at java.base/java.util.concurrent.ThreadPoolExecutor$AbortPolicy.rejectedExecution(ThreadPoolExecutor.java:2065)
	at java.base/java.util.concurrent.ThreadPoolExecutor.reject(ThreadPoolExecutor.java:833)
	at java.base/java.util.concurrent.ThreadPoolExecutor.execute(ThreadPoolExecutor.java:1365)
	at java.base/java.util.concurrent.AbstractExecutorService.submit(AbstractExecutorService.java:123)
	at java.base/java.util.concurrent.Executors$DelegatedExecutorService.submit(Executors.java:748)
	at org.opensearch.dataprepper.plugins.sink.opensearch.OpenSearchSink.doInitializeInternal(OpenSearchSink.java:254)
	at org.opensearch.dataprepper.plugins.sink.opensearch.OpenSearchSink.doInitialize(OpenSearchSink.java:2

Check List

• New functionality includes testing.
• New functionality has a documentation issue. Please link to it in this PR.
  • New functionality has javadoc added
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[dlvenable]

A bug fix like this should have a unit test that reproduces the issue along with the fix.

[graytaylor0]

Added unit test coverage and confirmed without the change the test will fail

Addressed comments