certificate-service: fix clang-tidy warnings

```
../redfish-core/lib/certificate_service.hpp:154:32: error: 'i' is an unsafe pointer used for buffer access [-Werror,-Wunsafe-buffer-usage]
    std::string_view::iterator i = value.begin();
```

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: I6b6fb17d58bb7a3bd861aaf020540586490693da