Allow using native free-text search (FTS) triplestore facilities

[berezovskyi]

Most vendors have a custom full-text search (FTS) implementation and SPARQL filters. Eg:

• http://docs.openlinksw.com/virtuoso/rdfsparqlrulefulltext/ (bif:contains)
• https://jena.apache.org/documentation/query/text-query.html (text:query)
• https://docs.marklogic.com/guide/semantics/semantic-searches#id_46386 (cts:contains)

We need to allow these features to be used independent of the rest of the Store features.

[jadelkhoury]

We also need to provide FTS combined with the oslc.where filtering.