Solr Index Metatag:robots Field cannot be indexed

There was a plugin in 8.x-1.x that allowed each individual meta tag to be indexed. It worked but required each tag to be added individually, rather than making the text of all meta tags added in a single chunk of text. I had not considered the use case of wanting to add them one at a time so it was removed.

Let's add the plugin from #2901039: Add a TypedData plugin to support Search API → but improve it.

I've added an updated version of https://www.drupal.org/project/metatag/issues/2901039#comment-13147190 → allowing to index individual meta tags, that works with metag 2.0.x.