Add continuum-schema-instance: build JSON instances from a JSON Schema

The Rust equivalent of json-schema-faker, scoped to what's actually
useful in Rust: resolves $ref (relative to whichever file references
it)/allOf, fills const/enum/default, and builds a value only for
properties listed in required - everything else is left out. For code
that has to construct a raw serde_json::Value against a schema it
doesn't own, instead of a typed struct.

Verified against continuum-schemas' real bambulab schemas manually, and
against local fixtures in tests (round-tripped through the jsonschema
crate to confirm the output actually validates).

packages/rust-schema-instance/tests/fixtures/events/item_created.schema.json

@@ -0,0 +1,15 @@
+{
+  "$schema": "https://json-schema.org/draft/2020-12/schema",
+  "title": "ItemCreatedEvent",
+  "allOf": [
+    { "$ref": "../common/envelope.schema.json" },
+    {
+      "type": "object",
+      "properties": {
+        "type": { "const": "item_created" },
+        "item_id": { "type": "string" }
+      },
+      "required": ["item_id"]
+    }
+  ]
+}