feat(input_schema): Support default, prefill and example in sub-properties (#555)

This PR:

- Adds support for specifying `default`, `prefill` and `example` for
sub-properties in input schema.
- change in apify-core is required to make it load the values, PR ready
here: apify/apify-core#23714
- parent property has priority over child property (but this is solved
in the apify-core) example:
  ```
  {
   "object": {
    "default": { "key1": "fromParent" },
    "properties": {
     "key1": {
       "default": "fromChild"
     }
    }
   }
  }
  ```
  Default would be: `{ "object": { "key1": "fromParent" } }`

- Adds support for specifying validation keywords under `items`
property, like:

  ```
  {
   title: 'Field title',
   description: 'My test field',
   type: 'array',
   editor: 'schemaBased',
    items: {
     type: 'string',
     pattern: '^\\d+$',
     minLength: 2,
     maxLength: 4
   }
  }
  ``` 
  This is handled automatically by `Ajv`.

Author: mfori