Split the Elasticsearch output and the Elasticsearch client

[ph]

Linked to #10547 and #10491

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We have found a few issues concerning output reloading triggered by a CM changes, the problem is we have a hard time to make the fix because the code of the Elasticsearch output is also used in other places in the code base for servers request, like uploading templates, uploading pipeline, checking for license.

We have decided to do the following steps:

• Extract the API layer in a esclientleg AKA Legacy
• Simplify the Elasticsearch output by only having the related code for publishing events and closing the output.
• Give us a migration path to the official ES golang client.

[andresrc]

@ycombinator what's missing to close this one?

[ycombinator]

This is done now, via #16150.