Change the client assertion signing to work in FIPS mode

[kc13greiner]

Summary

Closes
#270890

[infra-vault-gh-plugin-prod]

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[kibanamachine]

💔 Build Failed

• Buildkite Build
• Commit: 50e8d20
• Build duration: 98 mins

Failed CI Steps

• FTR Configs #7
• FTR Configs #80
• FTR Configs #80
• FTR Configs #106
• FTR Configs #140
• FTR Configs #145
• FTR Configs #153

Test Failures

• [job] [logs] FTR Configs #145 / Options list control "before all" hook: setup in "Options list control"
• [job] [logs] FTR Configs #80 / visualize app visualize listing page create and delete "before all" hook for "create new viz"
• [job] [logs] FTR Configs #80 / visualize app visualize listing page create and delete "before all" hook for "create new viz"

Metrics [docs]

✅ unchanged

History

cc @kc13greiner

[lorenabalan]

Change looks good to me, can we capture it in a unit test?
How come the test failures didn't fire on the PR that introduced the file? 🤔 I'm not ITAR compliant so I don't think I can test FIPS/FedRamp/etc. pipelines myself without CI running for me.