Prefer set with copy_from

[joegallo]

Proposed commit message

Prefer the copy_from option of the set processor for high certain high volume integrations.

When the field to be copied is already just a string, and so the set processor with mustache isn't being used for the side effect of converting to a string, then it's quite a bit faster to use copy_from rather than value (with mustache templating).

For example, in a large cluster that I was looking at a few minutes ago, the most expensive single set processor is this one:

{
  "set": {
    "field": "cloud.account.id",
    "if": "ctx.aws?.vpcflow?.account_id != null",
    "value": "{{aws.vpcflow.account_id}}"
  }
}

It's taking 2.8 microseconds per doc, as compared to the average of all set processor invocations for the same pipeline which is only .6 microseconds per doc. The cluster in question is processing billions and billions of documents per hour, though, so microseconds add up (and this particularly-expensive set processor is the eighth-most expensive processor for the entire pipeline).

---

I'm marking this a draft because I'm not 100% absolutely sure about my changes or the correct process for PRs on this repo.

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[joegallo]

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[andrewkroh]

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