Download a root certificate from the server and save it to a specified file path.
The path argument should include the file name and extension where the certificate will be stored.
For example, `/path/to/root.crt`. The recommended file extension for certificates is `.crt`.

alpacon authority download-crt [AUTHORITY ID] [flags]

  alpacon authority download-crt [AUTHORITY ID] --out=/path/to/root.crt

  -h, --help         # help for download-crt
  -o, --out string   # path where root certificate should be stored