Introduce vcpkg for dependency management

* Manually including libraries into the project while a perfectly acceptable solution
  leads to slow updates and large maintainance costs. To address this
  shortcoming we are introducing a package manager namely vcpkg into the
  codebase to handle the download and build of most external dependencies. Conan
  was also considered but vcpkg was chosen due to the centralized nature
  of the project. To achieve the integration the CMake configuration has
  been altered but not by much:

  1. Packages handled by vcpkg have been moved from
     externals/CMakeLists.txt to the root one to make them available to
     all subdirectories.

  2. In addition now find_package is used in accordance to the vcpkg
     recommendations.

  3. All libraries built by vcpkg have been updated to their latest
     available version, except for Qt because our codebase isn't
     compatible with Qt 6.0 yet.

  Currently there are build issues caused by the fmtlib upgrade.
  Another question is how to handle large libraries like Qt, Boost and
  SDL2 which will be addressed in a later commit.

src/web_service/announce_room_json.cpp

@@ -3,7 +3,7 @@
 // Refer to the license.txt file included.
 
 #include <future>
-#include <json.hpp>
+#include <nlohmann/json.hpp>
 #include "common/detached_tasks.h"
 #include "common/logging/log.h"
 #include "web_service/announce_room_json.h"