fix types for Result::kind and Result::level

[woodruffw]

This PR tweaks the serde-sarif build.rs script slightly to introduce two new rewrites:

• Result::kind is rewritten from Option<serde_json::Value> to Option<ResultKind>
• Result::level is rewritten from Option<serde_json::Value> to Option<ResultLevel>

This is done because the underlying code generator (schemafy) has only limited enum support, and fails to produce these enums (ResultKind and ResultLevel) on its own. This library instead has its own defined versions of each, which then don't get used in the generated code.

There are a handful of other places where the generated code currently uses serde_json::Value, but I these two are the only ones currently causing issues for me 🙂 -- I'd be happy to send a more generalized patch if desired, however.

Longer term, a more general fix here might be switch from schemafy (which is unmaintained) to typify, which has better enum support. I didn't attempt that in this PR, since I think it'd be a much larger undertaking.

Fixes #738.

[woodruffw]

NB: I've updated the API usage as well -- there were a handful of places where string -> type conversions were being performed, and these are no longer necessary.