Based on #19708.
This is on the path to porting the entire event class to Rust, as
`event.content` will then return the new Rust class `JsonObject`.
This PR adds a pure Rust `JsonObject` class that is a `Mapping`
representing a json-style object. It uses `serde_json::Value` as its
in-memory representation and `pythonize` for conversion when a field is
looked up on the object.
I'm not thrilled with the name, but couldn't think of a better one.
This also adds `JsonObject` handling to the JSON serialisation functions
we use, as well as to the `freeze(..)` function.
Reviewable commit-by-commit.
Erik Johnston