Add option to skip the index creation/index check

[yeikel]

In our cluster, we pre-create the index with custom mappings, analyzers, and other configurations.

Currently, the connector always attempts to create the index, which is unnecessary in our setup:

kafka-connect-elasticsearch/src/main/java/io/confluent/connect/elasticsearch/ElasticsearchSinkTask.java

Lines 240 to 244 in d394475

	private void ensureIndexExists(String index) {
	if (!indexCache.contains(index)) {
	log.info("Creating index {}.", index);
	client.createIndexOrDataStream(index);
	indexCache.add(index);

Although it does not create it again, it does issue an unnecessary call(and generates confusing logs) to our cluster

kafka-connect-elasticsearch/src/main/java/io/confluent/connect/elasticsearch/ElasticsearchClient.java

Lines 408 to 412 in d394475

	GetIndexRequest request = new GetIndexRequest(index);
	return callWithRetries(
	"check if index " + index + " exists",
	() -> client.indices().exists(request, RequestOptions.DEFAULT)
	);

The connector should be configurable to assume the index already exists and skip index creation attempts.