How to set a shared-library globally using the JCAC plugin?
https://github.com/jenkinsci/configuration-as-code-plugin/blob/master/demos/jenkins/jenkins.yaml

It should be possible by configuring it like this:

unclassified:
  globalLibraries:
    libraries:
      - name: "some-lib"
        retriever:
          modernSCM:
            scm:
              git:
                remote: "https://github.com/some-project.git"
                credentialsId: 'some-credentials'