Add support for credentials of type certificate

[ysmaoui]

Adding support to credentials of type certificate

withCredentials([
        certificate(
            credentialsId: certificateCredentialsId,
            keystoreVariable: 'PATH_TO_KEYSTORE',
            passwordVariable: 'PASSWORD'
        )
    ]) {
      sh(
        script: """
            curl -X POST \
                --cert-type P12 \
                --cert "\$PATH_TO_KEYSTORE":\$PASSWORD \
                ${url}
        """
      )
    }

Testing done locally with a certificate stored base64-encoded in an azure vault

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