Create a plugin manager for creating rankings.

Problem/Motivation

The current event subscriber has a hard coded ranking/query implementation. It would be ideal to instead call a custom plugin that handles this instead so developers can implement their own ranking/querying because it will vary site to site. The plugin could be a setting at the index level.

The current implementation can serve as the initial plugin, an example:

/**
 * Pure vector search ranker.
 */
#[SolrRanker(
  id: 'pure_vector',
  label: new TranslatableMarkup('Pure Vector'),
)]
class PureVector extends SolrRankerPluginBase {

  /**
   * {@inheritdoc}
   */
  public function applyRanking(SolariumQueryInterface $query, string $solr_field, int $top_k, array $vectors): SolariumQueryInterface {
    return $query->setQuery('{!knn f=' . $solr_field . ' topK=' . $top_k . '}[' . implode(', ', $vectors[0]) . ']');
  }

}