[Heartbeat] Enable script processor

[andrewvc]

Enables the script processor for heartbeat. This is an opt-in processor, but there's really never been a concrete need for that. This fixes #22788 .

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have made corresponding change to the default configuration files
• I have added tests that prove my fix is effective or that my feature works
• I have added an entry in CHANGELOG.next.asciidoc or CHANGELOG-developer.next.asciidoc.

How to test this PR locally

Add a script section to the processors block, it will work!

Use cases

Mostly useful for situations where people want to apply novel complex logic. See #22788

[elasticmachine]

Pinging @elastic/uptime (Team:Uptime)

[elasticmachine]

💚 Build Succeeded

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Build stats

• Build Cause: Pull request #23229 updated

• Start Time: 2020-12-21T22:53:53.323+0000

• Duration: 22 min 42 sec

Test stats 🧪

Test	Results
Failed	0
Passed	953
Skipped	13
Total	966

💚 Flaky test report

Tests succeeded.

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Test stats 🧪

Test	Results
Failed	0
Passed	953
Skipped	13
Total	966

[botelastic]

Hi!
We just realized that we haven't looked into this PR in a while. We're sorry!

We're labeling this issue as Stale to make it hit our filters and make sure we get back to it in as soon as possible. In the meantime, it'd be extremely helpful if you could take a look at it as well and confirm its relevance. A simple comment with a nice emoji will be enough :+1.
Thank you for your contribution!

[blakerouse]

Looks good.