Optionally leverage previews for progressive rendering

Motivation

Let's allow users to select the progressive rendering style. When rendering custom elements in a traditional Drupal context, atm it defaults to "Custom Elements Markup" rendering.
However, this only gives you a rendered presentation of your components if you added libraries to render it manually. It would be nice if one could use e.g. nuxt-previews there, when configured.

Proposal

1. Extend API to request varying rendering styles

\Drupal\custom_elements\CustomElement::toRenderArray() should receive some optional arguments, to select one of the following render-variants:
- Default value: Respect the configuration.
Optional, ask for some specific rendering strategy, one of:
- Custom Elements Markup
- Preview (automatic selection)
- Optional, specific preview provider (name of preview provider service, e.g. "markup")

2. Make a new configuration setting for "Progressive rendering". Allow selection of
- Custom Elements Markup
- Preview Provider (as it is selected automatically).

3. Let's improve lupus-decoupled to specifically ask for "custom elements markup" for its API requests, when markup rendering is requested.