// valid for.
  // See:
  // 	https://tools.ietf.org/html/rfc5280#section-4.2.1.3
  // 	https://tools.ietf.org/html/rfc5280#section-4.2.1.12
  //
  // Valid values are:
  //  "signing",
  //  "digital signature",
  //  "content commitment",
  //  "key encipherment",
  //  "key agreement",
  //  "data encipherment",
  //  "cert sign",
  //  "crl sign",
  //  "encipher only",
  //  "decipher only",

  //
  // Requests for TLS client certificates typically request: "digital signature", "key encipherment", "client auth".
  //
  // Requests for TLS serving certificates typically request: "key encipherment", "digital signature", "server auth".
  //
  // Valid values are:
  //  "signing", "digital signature", "content commitment",
  //  "key encipherment", "key agreement", "data encipherment",