Make the document store replaceable

It should be possible to select for each collection where the documents should be stored. The current behavior should be an "In the Collection" option. Extensions could, e.g., implement to directly query the documents to be indexed from an external data source or existing XWiki documents. The focus will be on reading documents. Updating documents through the REST API will only be available for the "In the Collection" option.

To implement this, a component role for a document store should be introduced. It should also be clearly documented how the document store informs the indexer about created, updated or changed documents.