Add an option to generate ECDSA certs for new devices

`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.

Start at the `az iot hub device-identity create` command and trace how it generates self-signed RSA certificates. Determine where an algorithm option belongs and how ECDSA certificates should be selected; done means the command can generate a valid ECDSA certificate without breaking the existing RSA behavior.

Written by the indexing model from the issue text.