This change adds a new authority option that allows to pass a callback
that returns the certificate chain and signer used to sign X.509
certificates.
This option will be used by Caddy, they renew the intermediate
certificate weekly and there's no other way to replace it without
re-creating the embedded CA.
Fixes#874
CloudKMS keys signs data using an specific signature algorithm, in RSA keys,
this can be PKCS#1 RSA or RSA-PSS, if the later is used, x509.CreateCertificate
will fail unless the template SignatureCertificate is properly set.
On contrast, AWSKMS RSA keys, are just RSA keys, and can sign with PKCS#1 or
RSA-PSS schemes, so right now the way to enforce one or the other is to used
templates.
