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

[efd6]

Replay of #37772 for the CEL input.

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.

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git merge upstream/main
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