Added checkout_cart_product_add_before event

[jignesh-baldha]

This PR will add checkout_cart_product_add_before event.

Fixed Issues (if relevant)

1. Add checkout_cart_product_add_before event #17830: Add checkout_cart_product_add_before event

Contribution checklist

• Pull request has a meaningful description of its purpose
• All commits are accompanied by meaningful commit messages
• All new or changed code is covered with unit/integration tests (if applicable)
• All automated tests passed successfully (all builds on Travis CI are green)

[magento-engcom-team]

Hi @jignesh-baldha. Thank you for your contribution
Here is some useful tips how you can test your changes using Magento test environment.
Add the comment under your pull request to deploy test or vanilla Magento instance:

• @magento-engcom-team give me test instance - deploy test instance based on PR changes
• @magento-engcom-team give me {$VERSION} instance - deploy vanilla Magento instance

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[okorshenko]

Hi @jean-bernard-valentaten thank you for your contributions. Unfortunately, all new features/improvements/code cleanup should go to the 2.3-develop branch. Only critical bug fixes can be delivered to 2.2-develop branch.
Just FYI: there is PR to 2.3-develop branch with the similar improvement. #18080