[9.2](backport #47441) x-pack/filebeat/input/{cel,httpjson,internal/dpop}: add support for DPoP OAuth for Okta

[mergify]

Proposed commit message

See title.

Note

This is initially included in the inputs/internal directory. It may be moved in future if it is needed for other packages that cannot see that import path. The API is unstable at this stage, with planned changes (dependent on the json/v2 experiment outcomes).

Note

Claude was used for some code generation in this PR.

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

Author's Checklist

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How to test this PR locally

Related issues

• For [Okta]: Support DPoP (Demonstrating Proof of Possession) Header in OAuth2 integrations#15056
• Closes x-pack/filebeat/input/{cel,httpjson}: add support for DPoP OAuth for Okta #47442

Use cases

Screenshots

Logs

---

This is an automatic backport of pull request #47441 done by [Mergify](https://mergify.com).

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🔍 Preview links for changed docs

• docs/reference/filebeat/filebeat-input-cel.md
• docs/reference/filebeat/filebeat-input-httpjson.md