[8.13](backport #38405) x-pack/filebeat/input/{cel,httpjson}: fix PEM key validation

[mergify]

Proposed commit message

Previously the validation was attempting to parse the PEM text as a key and was also attempting to parse the data as the wrong kind of key.

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 — also tested manually against the Okta integration as at okta: allow users to provide private key as a PEM block integrations#9291
• I have added an entry in CHANGELOG.next.asciidoc or CHANGELOG-developer.next.asciidoc.

Author's Checklist

• [ ]

How to test this PR locally

Related issues

• For [Okta] Supporting credentials in PEM format  integrations#8896

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Logs

---

This is an automatic backport of pull request #38405 done by [Mergify](https://mergify.com).

[botelastic]

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[mergify]

This pull request is now in conflicts. Could you fix it? 🙏
To fixup this pull request, you can check out it locally. See documentation: https://help.github.com/articles/checking-out-pull-requests-locally/

git fetch upstream
git checkout -b mergify/bp/8.13/pr-38405 upstream/mergify/bp/8.13/pr-38405
git merge upstream/8.13
git push upstream mergify/bp/8.13/pr-38405

[elasticmachine]

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• Start Time: 2024-03-19T06:53:41.912+0000

• Duration: 134 min 27 sec

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Passed	3330
Skipped	180
Total	3510

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