Blocks: Update schema to require either a type or an enum

Part of #35902.

Update the block.json schema to require either a `type` or an `enum` prop.

schemas/json/block.json

@@ -139,7 +139,14 @@
 							"description": "TA block attribute can contain a default value, which will be used if the type and source do not match anything within the block content.\n\nThe value is provided by the default field, and the value should match the expected format of the attribute."
 						}
 					},
-					"required": [ "type" ]
+					"anyOf": [
+						{
+							"required": [ "type" ]
+						},
+						{
+							"required": [ "enum" ]
+						}
+					]
 				}
 			},
 			"additionalProperties": false