feat: independent property graphs and graph writes from the client

A property graph definition now carries its mode. Independent graphs are defined by node and edge types with Arrow schemas and take rows through insert_into_property_graph; materialized view graphs are defined over tables as before and brought up to them with refresh_property_graph. rollback_property_graph returns either to its previous commit, and descriptions report the mode and the current and previous commits.

python/src/graph.rs

@@ -0,0 +1,27 @@
+// SPDX-License-Identifier: Apache-2.0
+// SPDX-FileCopyrightText: Copyright The LanceDB Authors
+
+//! Property graph schemas, between pyarrow and the JSON form a graph's
+//! definition carries.
+
+use arrow::{datatypes::Schema, pyarrow::PyArrowType};
+use lance_namespace::models::JsonArrowSchema;
+use pyo3::{PyResult, exceptions::PyValueError, pyfunction};
+
+/// A pyarrow schema as the JSON Arrow schema of a node or edge type.
+#[pyfunction]
+pub fn graph_schema_to_json(schema: PyArrowType<Schema>) -> PyResult<String> {
+    let json = lance_namespace::schema::arrow_schema_to_json(&schema.0)
+        .map_err(|error| PyValueError::new_err(error.to_string()))?;
+    serde_json::to_string(&json).map_err(|error| PyValueError::new_err(error.to_string()))
+}
+
+/// A node or edge type's JSON Arrow schema as a pyarrow schema.
+#[pyfunction]
+pub fn graph_schema_from_json(json: String) -> PyResult<PyArrowType<Schema>> {
+    let json: JsonArrowSchema =
+        serde_json::from_str(&json).map_err(|error| PyValueError::new_err(error.to_string()))?;
+    let schema = lance_namespace::schema::convert_json_arrow_schema(&json)
+        .map_err(|error| PyValueError::new_err(error.to_string()))?;
+    Ok(PyArrowType(schema))
+}