Add 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.

Would the primary use case for this be in controllers and form builders?

In which case I think a trait and adding that trait to FormBase and ControllerBase (that already have the request) feels reasonable.

The alternative is we add a new argument resolver to http_kernel.controller.argument_resolver and a HtmxHelper value object that can resolve from the current request.

That would allow people to typehint their controllers with `HtmxHelper $helper` and it would magically resolve like you can do for Request.

Left some minor comments on the MR.

Two questions on the MR.

I the plan to introduce the trait here and then add it to form and controller etc later?

Can we create follow ups for those with some notes for how to test it in deeper requests?

I think this is good, @fathershawn pointed out I missed this was already added to FormBase

Just one comment on the MR, fine to self RTBC after that I think

Hey a HTMX issue I can commit, yay!

Committed 5edaa75 and pushed to 11.x. Thanks!