Add parser of simplified sparql

[VanyaGlazunov]

This PR adds SPARQL query parsing capabilities to pathrex using the spargebra crate. We are trying to support SELECT queries with a single triple pattern in the WHERE clause. For example:

 SELECT ?x ?y WHERE { ?x <knows> ?y . }
 SELECT ?x ?y WHERE { ?x <knows>/<likes>* ?y . }
 SELECT ?x WHERE { <http://example.org/alice> <knows>+ ?x . }

spargebra is an excellent crate that provides production ready parser but we have to add some extra processing after we applied their parser.

1. Query Validation: We validate that the parsed query is a SELECT query (not ASK, CONSTRUCT, or DESCRIBE).

2. Pattern Extraction: We extract exactly one triple or property path pattern from the WHERE clause. Queries with multiple patterns or complex BGPs are rejected.

3. Desugaring Sequences: spargebra desugars sequence paths like ?x <a>/<b>/<c> ?y into a chain of BGP triples with intermediate blank nodes. We reconstruct these back into a single PropertyPathExpression::Sequence for our RPQ evaluators.

4. Subject/Object Validation: We ensure subjects and objects are either variables or named nodes (literals and blank nodes are not supported as they don't make sense for RPQ evaluation).

The pr closes #2

[VanyaGlazunov]

Hello @suvorovrain and @georgiy-belyanin can you look at this please