Add event description meta and update existing filter to allow for more flexible meta creation

[sdunham]

This PR makes two small updates:

1. It adds an invintus_event_description meta key to the default list of meta created for Invintus events imported via webhooks.
2. It adds the raw webhook event data array as an additional argument to the existing invintus/data/prepare filter

The intention behind the second update is to provide a more flexible method for developers to add additional meta fields derived from Invintus webhook data using meta_input argument. A hypothetical example:

// This would add my target key from the Invintus event data to the post meta
add_filter( 'invintus/data/prepare', function( $data, $event ){
    if (isset($event['some_invintus_key'])) {
        $data['meta_input'] = [
            'some_invintus_key' => $event['some_invintus_key']
        ];

    }
    
    return $data;
} , 10, 2 );