Add verbose output to troubleshoot certificate chain loading

[BryceDFisher]

In order to better troubleshoot certificate chain issues, it would be very helpful to have verbose output that states which certificates are being loaded as roots and which are being loaded as intermediaries. Logging the Subject would be sufficient for this purpose.

notation-cose/pkg/cose/verifier.go

Line 117 in 7bda470

for _, cert := range certs[1:] {

notation-cose/cmd/notation-cose/verify.go

Line 61 in 7bda470

for _, cert := range bundledCerts {

It would also be useful to log the filepath being used to load root certs.

notation-cose/cmd/notation-cose/verify.go

Line 56 in 7bda470

bundledCerts, err := cryptoutil.ReadCertificateFile(certPath)