feat(data-events): current_event() method

[miguelpeixe]

Make the current event name available during the handler execution.

All Submissions:

• Have you followed the Newspack Contributing guideline?
• Does your code follow the WordPress' coding standards and VIP Go coding standards?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?

Changes proposed in this Pull Request:

Allows for a handler to always be aware of which data event action it's under.

Take this example:

Data_Events::register_handler( 'create_contact', 'reader_registered' );
Data_Events::register_handler( 'update_lists', 'reader_subscribed' );

function create_contact( $timestamp, $data, $client_id ) {
  // Some data manipulation.
  sync( $contact );
}
function update_lists( $timestamp, $data, $client_id ) {
  // Some data manipulation.
  sync( $contact );
}

function sync( $contact ) {
  $contact = apply_filters( 'sync_contact', $contact );
  do_sync( $contact );
}

add_filter(
  'sync_contact',
  function( $contact ) {
    if ( 'reader_registered' === Data_Events::current_event() ) {
      // Some data manipulation only if it's called under "reader_registered"
    }
  }
);

How to test the changes in this Pull Request:

Other information:

• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes, as applicable?
• Have you successfully ran tests with your changes locally?