Add Strict Typing for all PHP code added in the scope of MSI poject

[maghamed]

Based on the Community Poll we've conducted
https://twitter.com/iminyaylo/status/905049975175962624

it makes sense to add declare(strict_types=1); for all PHP code in the scope of MSI

[naydav]

Also, need to add typecasting to Controllers and DataProviders

Example
https://github.com/magento-engcom/msi/blob/develop/app/code/Magento/Inventory/Controller/Adminhtml/Source/Edit.php#L51
In this case $sourceId is string

But expected type is integer
https://github.com/magento-engcom/msi/blob/develop/app/code/Magento/InventoryApi/Api/SourceRepositoryInterface.php#L51

After enabling Strict typing we will have got \TypeError
So need to add typecasting to all Controllers and DataProviders