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/Cargo.toml

@@ -0,0 +1,12 @@
+[package]
+name = "continuum-schema-instance"
+version = "0.1.0"
+edition = "2021"
+description = "Builds a JSON instance from a JSON Schema - fills const/enum/default, only builds required properties, everything else is left out"
+
+[dependencies]
+serde_json = "1"
+thiserror = "1.0"
+
+[dev-dependencies]
+jsonschema = "0.53.0"