[OpenSSH] Convert putty PPK key to OpenSSH format

To convert a putty-generated PPK key to the OpenSSH format, you can use puttygen:

puttygen MY_PPK_KEY.ppk -O private-openssh -o MY_OpenSSH_KEY

to convert a private key and:

puttygen MY_PPK_KEY.ppk -O public-openssh -o MY_OpenSSH_KEY

to convert a public key