WpSpaJson APIs

[arcnmx]

introduced in 0.4.8, these types (SpaJson, SpaJsonBuilder, SpaJsonParser) haven't really been fleshed out or used yet.

• [ ] implement manual methods that gir missed, basic constructors, etc
• [ ] create an example use-case for them, whether that's an actual example or just a set of #[test]s
• [ ] expose higher-level rust interfaces, etc
• [ ] improve conversion to/from them via serde or something if possible?
• [ ] consider also how they relate to libspa (though I think it entirely uses macros so isn't relevant or useful here?)