chore: add pydantic as an optional dependency

Author: georgeh0

pyproject.toml

@@ -67,6 +67,7 @@ dev = ["pytest", "pytest-asyncio", "ruff", "mypy", "pre-commit"]
 embeddings = ["sentence-transformers>=3.3.1"]
 colpali = ["colpali-engine"]
 lancedb = ["lancedb>=0.25.0"]
+pydantic = ["pydantic>=2.11.9"]
 
 # We need to repeat the dependency above to make it available for the `all` feature.
 # Indirect dependencies such as "cocoindex[embeddings]" will not work for local development.