Add an SSH key as a file in multipart/form-data.

Descriptive properties should all have the same name as those in AddSshKey, i.e. Name, Description, Passphrase, etc.
Bodyargs documentation will show JSON format, however it must be multipart/form-data and no PrivateKey
field is needed since the key will be read from the "File" key in the request.

        If Name is not specified, a default name will be generated:
          'username keytype, timestamp'