inputs.elasticsearch_query: Migrate unit test code to official client

[srebhan]

Migrate the unit test code away from the unmaintained library to the official ES client (github.com/elastic/go-elasticsearch/v5 and github.com/elastic/go-elasticsearch/v6).

depends on #19188

[alespour]

I tried two variants of simpler helper for the tests. The helper just detects version and indexes documents, that is what tests use ES client for now. One is using one using official (I picked v6, because integration tests use ES v6 image) client and another using ES REST API.

Tested the helper indexing + querying in the extent tests need. Both helper variants work with all ES version from v5 to v9 , just with handling minor nuances. Since official (v6) client is rather low-level, the length and complexity is almost identical. Using version-matching official client approach, which is necessary/required for the plugin itself, would just add more duplicated code here.

I tried integration tests with REST-based indexer/ingester helper, they passed OK with both ES v5 and v6.

I lean towards using pure ES REST API based helper in the plugin's tests.

[srebhan]

Please don't use pure REST APIs! We need to maintain all of that and if people start doing this for the 300+ plugins we have we run into issues! Please use the official client!

For the versioning: If the official v5 client works for also later versions of the server it's fine to use it! We don't need to match the versions in the tests...

[alespour]

#19240 ready for review