Add possibility to exclude products from Nosto index

[supercid]

Add a filter / validation parameter to the events. The validation can be overwritten and set to false when Nosto will skip the product completely in tagging, indexing and in API call

[supercid]

Closed as has been resolved here:
https://github.com/Nosto/nosto-magento/wiki/Prevent-indexing-certain-products

[mridang]

@supercid Let's reopen this since we should actually dispatch a before event (similar to Magento2) and prevent the product from even being loaded.