parameter.schema.json: Add readOnly property and make type optional (#2449)

* parameter.schema.json: Add readOnly property and make type optional

Add a readOnly boolean property to the parameter metadata schema to
indicate parameters that should not be changed by the user.

Make the type field optional since it can be determined at runtime
from the PARAM_VALUE message.

* Apply suggestion from @hamishwillee

---------

Co-authored-by: Hamish Willee <hamishwillee@gmail.com>

component_metadata/parameter.schema.json

@@ -8,7 +8,7 @@
         "version": {
             "description":  "Version number for the format of this file.",
             "type":         "integer",
-            "minimum":      2
+            "minimum":      3
         },
         "translation": {
             "type": "object",
@@ -122,9 +122,14 @@
                                 }
                             }
                         }
+                    },
+                    "readOnly": {
+                        "description":  "true: value is read only and should not be changed by user.",
+                        "type":         "boolean",
+                        "default":      false
                     }
                 },
-                "required":             [ "name", "type" ],
+                "required":             [ "name" ],
                 "additionalProperties": false
             }
         }