Add a `search` parameter to the LDP container service

Modify `ldp.container.get` to accept an (optional) `search` parameter that would return only those container resources that contain the keyword.

Based on @simonLouvet's request:

```
{
SELECT ?s1
WHERE {
?s1 ?p1 ?o1 .
FILTER regex(str(?o1), “${filter.q}”)
FILTER NOT EXISTS { ?s1 a ?o1}
}
}
```

For the moment, do not make this parameter available via the API.

Start at the ldp.container.get service entry point and trace how container resources are selected. Use the supplied SPARQL query as the behavioral reference; done means an optional search value filters matching resources while the parameter remains unavailable through the API.

Written by the indexing model from the issue text.