[9.0](backport #44795) Adding the now processor

[mergify[bot]]

Adding the now processor to avoid having to resort to scripts to add the current timestamp

Proposed commit message

Adding a now processor so we can add the current timestamp to fields like event.created without having to resort to the script processor.

Checklist

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

Disruptive User Impact

None anticipated

Author's Checklist

to be discovered

How to test this PR locally

Create a pipeline and define the now processor as described in the docs

Related issues

• todo

Use cases

Adding timestamp to the generated event, adding timestamps as the event is forwarded to provide traceability throughout the pipeline

---

This is an automatic backport of pull request #44795 done by [Mergify](https://mergify.com).

[elasticmachine]

Pinging @elastic/elastic-agent-data-plane (Team:Elastic-Agent-Data-Plane)

[mjmbischoff]

I don't think this enhancement needs to be on the 9.0 (or feature frozen 9.1?) branch