iop: ssh_(install_key): Unify scripts and rework
Unify the iop-subcommands install_key and ssh_install_key to one script. In addition:
- Support new key types such as ssh-ed25519 which is supported by Dropbear nowadays. Also add unsupported ones to make them work automatically in the future.
- For the install_key command, do not re-add keys already added
- Read keys loaded into SSH Agent as well, which is useful when working with agent forwarding on a remote host or in a Docker container.
- For the ssh_install_key command also add keys added manually to the build dir.