feat: allow $schema entry in SPDX 2.3 JSON

Closes #864.

Signed-off-by: Maximilian Combüchen <[email protected]>

Author: mcombuechen

schemas/spdx-schema.json

@@ -4,6 +4,10 @@
   "title" : "SPDX 2.3",
   "type" : "object",
   "properties" : {
+    "$schema": {
+      "type": "string",
+      "description": "Reference the SPDX 2.3 JSON schema."
+    },
     "SPDXID" : {
       "type" : "string",
       "description" : "Uniquely identify any element in an SPDX document which may be referenced by other elements."