Clarify which format is used

[lairdm]

The API provides both PEM and x509, the naming of this routine is a slightly unclear on which file is expected. Based on the name I thought it would want me to provide the x509, not the .pem. Thanks.

aap-client-python/aap_client/crypto_files.py

Line 7 in d4814e4

def load_public_from_x509(x509_filename):

[psafont]

I'm adding a function to read DER certificates, as well as removing the private key loader from the API as to reduce the confusion.