add support for binary-encoded x509 certs

[sozercan]

notation verify only supports validating with ASCII-encoded x509 certs (PEM)

Today, you can convert a binary encoded DER to PEM and verify using PEMs:
openssl x509 -in mycert.crt -inform DER -out mycert.pem

However, users shouldn't need to do this conversion manually and notation should add support for binary-encoded x509 certs

[SteveLasker]

Tagging @qmuntal to see if you can pick this up. Our favorite issue hunter...

[patrickzheng200]

Taking a look.

[iamsamirzon]

With the new trust store implementation, where certificates can be added in either DER or PEM format, this issue can be closed.

[dtzar]

This is complete via the linked PR 235