Support loading key stores from PEM files

[jroper]

Currently, only trust stores can be effectively loaded from PEM files, since the PEM loader only attempts to load certificates, it does not attempt to load any private keys. However, consider a platform like Kubernetes, where both the certificate, and the server key, are provided to a pod in PEM format. Currently to load that into ssl-config, you need to first convert these into a PKCS12 or JKS key store, which is typically done using an init container with openssl installed, and is a significant amount of extra configuration and deployment complexity. If ssl-config could load the private key in PEM format too, then the supplied certificate and key could be consumed directly by ssl-config with no preprocessing step necessary.