[Data Liberation] Set WP_XML_Processor memory limit cap

[zaerl]

The default XML process memory budget is one GB: https://github.com/WordPress/wordpress-playground/blob/trunk/packages/playground/data-liberation/src/xml-api/WP_XML_Processor.php#L596

I think it's pretty generous as an amount and does not take into consideration memory_limit. We can make it customizable for the final user as an "advanced feature" and have a cap on current machine resources to avoid PHP "allowed memory size..." fatal error.