Add key basic_auth >username_file (SchemaStore#4846)

Author: ygreyeb

src/schemas/json/prometheus.json

@@ -40,6 +40,9 @@
         "password": {
           "type": ["string", "null"]
         },
+        "username_file": {
+          "$ref": "#/definitions/filepath"
+        },
         "password_file": {
           "$ref": "#/definitions/filepath"
         }