Add drs_uri to DCP/2 file descriptor

The immediate need is for LungMAP but this property could be useful in other contexts as it allows for files to be hosted by DRS implementations other than TDR.

json_schema/system/file_descriptor.json

@@ -94,6 +94,13 @@
       "user_friendly": "S3 ETag",
       "type": "string",
       "example": "c92e5374ac0a53b228d4c1511c2d2842-63"
+    },
+    "drs_uri": {
+      "description": "Data Repository Service URI pointing at this file. If this property is absent, a data file exists in the repository containing this descriptor. If this property is present and not `null`, a data file exists in another repository at the specified URI. If this property is present but `null`, the data file is not available, but will likely become available in the future, along with an updated descriptor referencing it.",
+      "user_friendly": "Data Repository Service URI",
+      "type": ["string", "null"],
+      "format": "uri",
+      "example": "drs://drs.example.org/314159"
     }
   }
 }