Add Support for Public Key Pemfiles

[holodorum]

To implement issue 423 in rustls we want to read Public Keys from pemfiles. In this pull request this functionality is implemented.

The value type of SubjectPublicKey is a pki_types::CertificateDer instead of SubjectPublicKeyInfoDer, initially I thought it would be more intuitive to use pki_types::SubjectPublicKeyInfoDer instead of CertificateDer. However, the ClientCertVerifier and ServerCertVerifier traits in rustls expect a CertificateDer struct so I stuck with that.
I'm not sure if there is ever a scenario where we want to load multiple Public Keys at once. If not it is most likely better to change the public_keys function into a public_key function and return an Option.