The motivation for this change is to prevent extensions from having to check executor globals for the current execute_data during function call init. A previous implementation of the observer API initialized the function call from runtime cache initialization before execute_data was allocated which is why zend_function was passed in.
But now that the observer API is implemented via opcode specialization, it makes sense to pass in the execute_data. This also keeps the API a bit more consistent for existing extensions that already hook zend_execute_ex.
Closes GH-6209
