Also fix some Javadoc errors and ensure Javadoc (and source) jar generation is tested on CI.

This also fundamentally changes the `Type` passed to the converter. Instead of just passing the runtime class, we use the type parsed from the method declaration. This allows complex types like `List<Record<Person>>` to be handled by the converter instead of just receiving something like `ArrayList`.

This change means that only RequestBody and ResponseBody are the types which are allowed for their respective objects by default. If you want serialization of any kind, you need to opt-in with either your own converter or one of the provided ones.

(cherry picked from commit 62903c4b)

(cherry picked from commit 2f4caa6f)