x-pack/filebeat/input/httpjson: add support for pem encoded keys

[efd6]

Proposed commit message

This adds a new Okta auth field, jwk_pem, that allows users to specify a PEM-encoded private key for authentication.

Also refactor the JSON-based code to simplify and add minimal testing.

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
• I have added an entry in CHANGELOG.next.asciidoc or CHANGELOG-developer.next.asciidoc.

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Related issues

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

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