Add a methods to get info about which elements are involved in an HTMX Request

@fathershawn,

Per our conversation at DrupalNYC Contribution day, i think it would be helpful to have a trait to parse the request headers. Something like...

trait HTMXHeaders {
  public function getHtmxTrigger(): ?string;
  public function getHtmxTriggerName(): ?string;
}

These functions can look for the request ($this->request ?: \Drupal->getRequest()) and then get the headers.

We also talked about the helpfulness of a getHtmxTriggerElement($build): ?array function which could locate the render element and to allow access to properties or data, but decided this wasn't critical at this time.