Add an option to generate ECDSA certs for new devices

[rojer]

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az iot hub device-identity create offers to generate self-signed certs for devices. however, those are RSA certs, and it would be nice to be able to generate ECDSA as well. they work just fine and have a number of advantages, one being that they are smaller, require less memory during handshake and EC math is faster than RSA. if not outright switching to ECDSA, at least an option would be nice.