Introduce p2p_get_connection_types_for_post()

Given that `connected_type` is required and 3rd-party plugins/themes won't always know of connection types, this function provides developers with a method of retrieving the connection types associated to a single post.

Usage as follows;
```
<?php
// Find connection types for this post
$connected_types = p2p_get_connection_types_for_post( get_queried_object()->ID );

// Find connected pages
$connected = new WP_Query( array(
  'connected_type' => $connected_types,
  'connected_items' => get_queried_object(),
  'nopaging' => true,
) );
```