Add a rel=type link header to Node and Media GET responses

[dannylamb]

Add a rel=type link header to Node and Media GET responses for each rdf:type defined in the rdf mapping for its respective bundle.

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"'],
  ];
}

Media don't have a specific alter, but you can use hook_entity_view_alter to achieve the same effect.