[8.19](backport #49768) Add helper methods to support kafka output in beatreceivers

[mergify]

Proposed commit message

This PR adds helper methods on fmtstr package to help parse dynamic fields. This is required to support kafka output on beatreceivers

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. Where relevant, I have used the stresstest.sh script to run them under stress conditions and race detector to verify their stability.
• I have added an entry in ./changelog/fragments using the changelog tool.

Disruptive User Impact

None

Related issues

• Relates https://github.com/elastic/ingest-dev/issues/7162

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This is an automatic backport of pull request #49768 done by [Mergify](https://mergify.com).

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