Add custom event encoder for Elasticsearch output

[fearful-symmetry]

What does this PR do?

Fixes #256
Fixes #253

This adds a custom Event type along with the fastjson implementation in order to correctly add a @timestamp field to events. This is something the Beats elasticsearch output already does, and presumably went unnoticed as the logic is buried in a special little encoder callback.

Why is it important?

We need a @timestamp field.

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have added tests that prove my fix is effective or that my feature works
• I have added an entry in CHANGELOG.md or CHANGELOG-developer.md.

[mergify]

This pull request does not have a backport label.
If this is a bug or security fix, could you label this PR @fearful-symmetry? 🙏.
For such, you'll need to label your PR with:

• The upcoming major version of the Elastic Stack
• The upcoming minor version of the Elastic Stack (if you're not pushing a breaking change)

To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-v8./d.0 is the label to automatically backport to the 8./d branch. /d is the digit

[elasticmachine]

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• Start Time: 2023-03-02T00:03:38.857+0000

• Duration: 17 min 59 sec

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