feat: add prefill option to input schema (#460)

related to https://github.com/apify/apify-core/issues/16271

I checked the tests and they seem to be quite generic, so it looks like
this case should be covered too

packages/input_schema/src/schema.json

@@ -243,6 +243,7 @@
                 "title": { "type": "string" },
                 "description": { "type": "string" },
                 "default": { "type": "boolean" },
+                "prefill": { "type": "boolean" },
                 "example": { "type": "boolean" },
                 "nullable": { "type": "boolean" },
                 "groupCaption": { "type": "string" },