fix: Update nlohmann-json.cmake to use FetchContent

Author: bobtista

cmake/nlohmann-json.cmake

@@ -0,0 +1,14 @@
+FetchContent_Declare(
+    nlohmann_json
+    GIT_REPOSITORY https://github.com/nlohmann/json
+    GIT_TAG        v3.12.0
+)
+
+FetchContent_GetProperties(nlohmann_json)
+if(NOT nlohmann_json_POPULATED)
+    FetchContent_Populate(nlohmann_json)
+    add_library(nlohmann_json INTERFACE)
+    target_include_directories(nlohmann_json INTERFACE
+        $<BUILD_INTERFACE:${nlohmann_json_SOURCE_DIR}/single_include>
+    )
+endif()