Compiler warning was signed / unsigned compare.

The `body` field of the Response is optional. Do not error if it is missing.

If the typeinfo of the response. This was incorrectly using the Request type.

Authored by kuafuwang, squashed by ben-clayton.

The onError parameter of Server::start was default initialized with
OnError(), which is an 'invalid target', not a no-op implementation.

Replace with a true no-op implementation.

`nlohmann/json` raises a `size_t` to `int` MSVC warning that will fail the build if `CPPDAP_WARNINGS_AS_ERRORS` is enabled.
Given that this is not our code, the best we can do is to make an exception for this warning.

Only set the third_party directory variables if they're not already set
(by the dependee project).