external: add nlohmann_json v3.11.2

Single header downloaded from https://github.com/nlohmann/json/releases/tag/v3.11.2

Add a custom CMakeLists.txt to have a `nlohmann_json::nlohmann_json`
target to link against (just like the full project)

external/nlohmann_json/CMakeLists.txt

@@ -0,0 +1,14 @@
+project(nlohmann_json LANGUAGES CXX VERSION 3.11.2)
+
+add_library(nlohmann_json INTERFACE)
+
+target_include_directories(nlohmann_json
+  SYSTEM INTERFACE
+    $<BUILD_INTERFACE:${CMAKE_CURRENT_LIST_DIR}/include>
+)
+# Enable extended diagnostics information
+# https://github.com/nlohmann/json/releases/tag/v3.10.0
+target_compile_definitions(nlohmann_json INTERFACE JSON_DIAGNOSTICS=1)
+
+# provide a namespaced alias for clients to 'link' against if nlohman_json is included as a sub-project
+add_library(nlohmann_json::nlohmann_json ALIAS nlohmann_json)