Prepare async index vulnerability_finding_signatures_on_signature_sha

What does this MR do and why?

This prepares for adding vulnerability_finding_signatures_on_signature_sha index asynchronously as per the guidelines.

The index is added to fix timeout issues with OverrideUuidsService.

Another merge request will follow to add the index synchronously after this is verified.

Resolves #397004 (closed).

How to set up and validate locally

Please follow this guide to validate the index was created asynchronously locally on your machine.

MR acceptance checklist

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