Introduce `JsonSafeString`

[nblumhardt]

Fixes #46

Since no serialization actually occurs, the PR proposes to do this without relying on the @ operator that the original ticket describes.

JsonSafeString can be passed anywhere that property values are accepted (including through BeginScope()).

Usage

var json = "{\"A\": 42}";
_logger.LogInformation("The answer is {Answer}", new JsonSafeString(json));

Result