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`.

certbot/CHANGELOG.md

@@ -6,7 +6,8 @@ Certbot adheres to [Semantic Versioning](https://semver.org/).
 
 ### Added
 
-*
+* `acme.messages.OrderResource` now supports being round-tripped
+  through JSON
 
 ### Changed