Add category (search index) field to Amazon Product Widget

The following patch adds a category / search index field to the Amazon Product Widget in order to be able to do the search in a specific category (e.g. Music, MoviesAndTV, Books, Shoes and more). The list of categories (search indizes) can be found here:
https://webservices.amazon.com/paapi5/documentation/locale-reference/germany.html

The patch is not perfect. Would be great if this feature could be added to one of the next releases.

Additionally to the patch the following database tables need to be expanded by the following column:

ALTER TABLE drupal_amazon_paapi_node__field_amazon_product_widget
ADD COLUMN `field_amazon_product_widget_search_index` varchar(255) DEFAULT NULL COMMENT 'Category / Search Index used for fetching products as a fallback when the provided ASINs are unavailable.';

ALTER TABLE drupal_amazon_paapi_node_revision__field_amazon_product_widget
ADD COLUMN `field_amazon_product_widget_search_index` varchar(255) DEFAULT NULL COMMENT 'Category / Search Index used for fetching products as a fallback when the provided ASINs are unavailable.';