Accept an encrypted key as client credential

[chlowell]

Documentation indicates keys should be provided to ConfidentialClientApplication as strings, PEM encoded. The documentation doesn't mention how to provide an encrypted key, and there's nothing obvious like a password parameter, so on the face of it ConfidentialClientApplication doesn't accept encrypted keys.

The implementation allows something of a workaround. It so happens that the given private key gets passed along to PyJWT, which will accept a private key as deserialized by cryptography. So this works today:

from cryptography.hazmat.primitives import serialization
private_key = serialization.load_pem_private_key(pem_bytes, password=password, backend=default_backend())
ConfidentialClientApplication(client_credentials={"private_key": private_key, ...})

But that relies on today's implementation and isn't a substitute for formal support.

[rayluo]

@abhidnya13 Let's add one more key (something like passphrase or private_key_passphrase?) into the existing client_credentials input parameter, and then internally we can use the implementation described above. Also, if the input private_key_passphrase is a string, we can support automatically convert it into bytes.