Metadata missing from the serialisation schema

[aborgna-q]

Rust defines a field for node metadata.

hugr/hugr/src/hugr/serialize.rs

Lines 56 to 59 in 9d2de07

	// TODO: Update to Vec<Option<Map<String,Value>>> to more closely
	// match the internal representation.
	#[serde(default)]
	metadata: Vec<serde_json::Value>,

But this is missing from the schema.

hugr/hugr-py/src/hugr/serialization/serial_hugr.py

Lines 12 to 17 in 9d2de07

	"""A serializable representation of a Hugr."""
	version: Literal["v1"] = "v1"
	nodes: list[OpType]
	edges: list[Edge]

While we are fixing this, we should probably address the TODO on the rust definition, and make the each node's metadata an Option<HashMap<String, _>>.