I try to create a certificate signed by another CA certificate file. I suppose CertificateParams.signed_by looks like what I need:

pub fn signed_by(
  self,
  public_key: &impl PublicKeyData,
  issuer: &Certificate,
  issuer_key: &KeyPair,
) -> Result<Certificate, Error>

But, I'm confused on how to get issuer from an existed CA pem file on my disk.

Or, is there any other way to create a certificate signed by existed CA certificate?

Thanks.