Add support for extension fields in v2.x and v3.4

Signed-off-by: Joffrey F <[email protected]>

compose/config/config_schema_v3.5.json

@@ -64,6 +64,7 @@
     }
   },
 
+  "patternProperties": {"^x-": {}},
   "additionalProperties": false,
 
   "definitions": {