Add bindings for csharp modules to use JWT claims

[jsdt]

Description of Changes

This exposes JWT claims for csharp modules, similar to how they are exposed to rust modules in #3288.

This adds the new types AuthCtx and JwtClaims, and adds an AuthCtx to the ReducerContext.

AuthCtx represents the credentials associated with the request, and JwtClaims represents a jwt token.

One difference from the rust version is that I didn't create helpers to build an AuthCtx from a jwt payload. The reason is that we would need to be able to compute the identity from the payload claims, which requires a blake3 hash implementation. The first two c# libraries I found had issues at runtime (Blake3 is wrapping a rust implementation, and HashifyNet seems to be broken by our trimming because it uses reflection heavily). I can look into taking the implementation from HashifyNet, since it is MIT licensed, but I don't think we need to block merging on that.

API and ABI breaking changes

This adds the new types AuthCtx and JwtClaims, and adds an AuthCtx to the ReducerContext.

This also adds a csharp wrapper for the get_jwt ABI function added in #3288.

Expected complexity level and risk

Testing

This has a very minimal unit test of JwtClaims.

I manually tested using this locally with the csharp quickstart, and I was able to print jwt tokens inside the module.

[rekhoff]

The changes/additions are clear and well commented; no objections. If we can get the comments addressed and the tests passing, that would be ideal, but I'll still mark my approval now.