Add a canonical JSON impl (#19739)

This comes from
https://github.com/erikjohnston/rust-signed-json/blob/main/src/json.rs.
We need to be able to serialise canonical JSON in Rust to be able to
calculate event IDs once we port the event class to Rust.

We could instead make the above a properly published crate, but feels
easier to pull it into Synapse utils.

Author: erikjohnston

Cargo.lock

@@ -0,0 +1 @@
+Add a Rust canonical JSON serializer.

changelog.d/19739.misc

@@ -44,7 +44,7 @@ pythonize = "0.27.0"
 regex = "1.6.0"
 sha2 = "0.10.8"
 serde = { version = "1.0.144", features = ["derive"] }
-serde_json = "1.0.85"
+serde_json = { version = "1.0.85", features = ["raw_value"] }
 ulid = "1.1.2"
 icu_segmenter = "2.0.0"
 reqwest = { version = "0.12.15", default-features = false, features = [
@@ -56,6 +56,7 @@ http-body-util = "0.1.3"
 futures = "0.3.31"
 tokio = { version = "1.44.2", features = ["rt", "rt-multi-thread"] }
 once_cell = "1.18.0"
+itertools = "0.14.0"
 
 [features]
 extension-module = ["pyo3/extension-module"]