acme.messages.OrderResource: Make roundtrippable through JSON (#9617)

Right now if you to_json() an `OrderResource` and later deserialize
it, the `AuthorizationResource` objects don't come back through the
round-trip (they just get de-jsonified as frozendicts and worse, they
can't even be passed to `AuthorizationResource.from_json` because
frozendicts aren't dicts). In addition, the `csr_pem` field gets
encoded as an array of integers, which definitely does not get
de-jsonified into what we want.

Fix these by adding an encoder to `authorizations` and encoder and
decoder to `csr_pem`.

AUTHORS.md

@@ -23,6 +23,7 @@ Authors
 * [amplifi](https://github.com/amplifi)
 * [Andrew Murray](https://github.com/radarhere)
 * [Andrzej Górski](https://github.com/andrzej3393)
+* [Anna Glasgall](https://github.com/aglasgall)
 * [Anselm Levskaya](https://github.com/levskaya)
 * [Antoine Jacoutot](https://github.com/ajacoutot)
 * [April King](https://github.com/april)