Fix native Facebook graph query to retrieve engagement counts

[NormanHoehne]

Description

I have added a new argument 'fields' to the Facebook api query to make it possible to retrieve count statistics again. Since the api response has changed, I have also updated the lines that assign the counts.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• Enhancement (modification of the currently available functionality)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project (WordPress coding standards, etc).
• My code has appropriate phpdoc comments with a description, @since, @params and @return.
• My code is tested for both new installs and for users that are upgrading from older versions.

[billerickson]

Thank you!