Allow overriding generic types #180

Allow using typedefof to override options for all instances of a generic type. Example:

let options =
    FSharpJsonOptions()
        .WithOverrides(fun o -> dict [ typedefof<Result<_, _>>, o.WithUnionTagName("Result") ])
        .ToJsonSerializerOptions()

JsonSerializer.Serialize((Ok "test": Result<string, string>), o)
// --> {"Result":"Ok","Fields":["test"]}

• Q: What should happen if a type is matched by both a generic and a specific override?
  A: Only the specific one is taken into account.

  Example:

  let options =
      FSharpJsonOptions()
          .WithOverrides(fun o -> dict [
              typedefof<Result<_, _>>, o.WithUnionTagName("GenericCase")
              typedof<Result<string, string>>, o.WithUnionTagName("SpecificFields")
          ])
          .ToJsonSerializerOptions()
  
  JsonSerializer.Serialize((Ok "test": Result<string, string>), o)
  // --> {"Case":"Ok","SpecificFields":["test"]}