Expose OBJ media as link header

[dannylamb]

Every time a node is viewed, check to see if it has field_obj, and if it does, add the url to referenced entity as a link header.

You have access to the node and can add link headers in hook_node_view_alter like so:

function islandora_node_view_alter(&$build, EntityInterface $node, EntityViewDisplay $display) {
  $build['#attached']['http_header'] = [
    ['Link', '<http://islandora.ca>; rel="awesome"'],
  ];
}

We don't have any defined rel types at the moment, so just use something like obj and we'll mint something official in a subsequent ticket.

[dannylamb]

Closing since I'm about to make a batch of tickets to supersede this and then some.