Add RDF merging functionality for multi-provider knowledge graphs

Implements the ability to merge RDF files from multiple Croissant providers
into a unified knowledge graph. This addresses issue #850 objective.

Features:
- Merge multiple RDF files with automatic deduplication
- Support for various RDF formats (Turtle, N-Triples, RDF/XML, etc.)
- CLI tool 'merge-rdf' for easy merging
- Wildcard support for batch merging (e.g., *.ttl)
- Output format selection (turtle, json-ld, n3, nt, xml)
- Comprehensive tests for merging and deduplication

Example:
  merge-rdf huggingface.ttl openml.ttl kaggle.ttl -o unified.ttl

Author: david4096