rfc(dialog): ability to specify renderer per open call

[bigopon]

This is a formal issue to discuss the ideas in #1671

At the moment, each dialog service is associated with only a single dialog-dom-renderer. This means that applications may need to create multiple dialog services to produce different dialog dom structures. For example: one for notifications and one for modals, because notifications normally have a different DOM structure compared to modals.
This inflexibility is unnecessary, and it would be better if the dialog service could have different renderer for different occasions:

class MyApp {
  alert() {
    dialogService.open({
      renderer: modalRenderer, // render standard modal dom with overlay, host etc...
      ...
    })
  }

  notify() {
    dialogService.open({
      renderer: notificationRenderer, // no need overlay
      ...
    })
  }

cc @ekzobrain