Add RSA-X931 (#868)

fixes #867 

However, I was unable to find a good link for the standard yet, so this
remains draft.

Author: stevespringett

schema/cryptography-defs.json

@@ -40,6 +40,21 @@
         }
       ]
     },
+    {
+      "family": "RSA-X931",
+      "standard": [
+        {
+          "name": "X9.31",
+          "url": "https://standards.globalspec.com/std/1955293/ansi-x9-31"
+        }
+      ],
+      "variant": [
+        {
+          "pattern": "RSA-X9.31[-{hashAlgorithm}][-{keyLength}]",
+          "primitive": "signature"
+        }
+      ]
+    },
     {
       "family": "RSAES-PKCS1",
       "standard": [