Api codegen: Full support of additional properties

[kevin-legion]

In the end of the day this pr fully supports the additionalProperties feature and not just some cases.

Here is what the codegen does when the value of additionalProperties is

• [x] true - map string -> any
• [x] false - noop
• [x] some schema - map string -> schema
• [x] true + properties is provided - struct + flatten __additional_properties: map string -> any
• [x] false + properties is provided - same as without the additionalProperties option
• [x] some schema - struct + flatten __additional_properties: map string -> schema

@kirdaybov The support for the Python is rather limited for now as I wasn't too sure what you'd like to generate.

@ereOn I had to compromise in Rust to find a type that could feat the "any" type. There are many constraints (Clone, Hash, Deserialize, PartialEq/Ord, etc...) and unfortunately a supertrait that'd require all the above constraints would not work (since Deserialize requires Self to be Sized). We could almost use serde_json::Value but it doesn't implement Hash (https://github.com/serde-rs/json/issues/720 and https://github.com/serde-rs/json/issues/747). So instead I created a new JsonValue newtype that wraps serde_json::Value and implements Hash (it's fine except for null).

What do you think?

[kevin-legion]

@ereOn I applied the changes discusses here and offline:

• [x] Delete StructMap
• [x] Make the derive(Hash) conditional