Add support for distributed search and shards

Pull-request: https://github.com/mkalkbrenner/search_api_solr/pull/94

Could you explain what use-case you try to implement?
Search API Solr already supports distributed searches (for automatic shards).

If you really want to use sharding to split data, for example per language, some more features like those your module provides, might be required, for example "routing".

But don't get me wrong. I'm interested in that.

Yes, I think the use case that would be better understood would be when you have several portals and you want to be able to display the content of all of them in addition to the content of the portal through which it is accessed in the same view (as a general search engine), being able to maintain compatibility with the facets and the autocomplete search engine, so that when you click on each of them, there is a redirect that sends you to the original content or is retrieved through API to represent it in the portal itself as if it were its own content.

In other words, content syndication.

When you have a core solr configured that queries other cores through shards, drupal is not able to retrieve the schema fields of all of them because it only retrieves the fields contained in the core, which in this case is 0 because the core only queries the content of other cores.

The functionality that I propose to integrate retrieves these shards to retrieve the schema fields of each of the configured cores. The interesting thing at this point would be to be able to transform or map the retrieved fields directly from the shards to drupal entities, although I do not think this is possible.