Introduce nlohmann/json as a third-party lib (#11151)

### Summary

With the goal of introducing CoreML AOT
(#9800), we need to
introduce
[runtime/inmemoryfs/inmemory_filesystem_utils.cpp](https://github.com/pytorch/executorch/blob/main/backends/apple/coreml/runtime/inmemoryfs/inmemory_filesystem_utils.cpp).
However, this [relies on
nlohmann/json](https://github.com/pytorch/executorch/blob/306646390734d3766a72751a6bcd79a4817a01ba/backends/apple/coreml/runtime/inmemoryfs/inmemory_filesystem_utils.cpp#L13)
— so let's just introduce it as a third-party library instead of doing a
hidden clone.

### Test plan

CI

Author: jathu

.gitmodules

@@ -67,3 +67,6 @@
 [submodule "shim"]
 	path = shim
 	url = https://github.com/facebook/buck2-shims-meta
+[submodule "third-party/json"]
+	path = third-party/json
+	url = https://github.com/nlohmann/json.git

backends/apple/coreml/runtime/workspace/executorchcoreml.xcodeproj/project.pbxproj

@@ -925,7 +925,7 @@
 					"$(SRCROOT)/../include/executorch/runtime/core/portable_type/c10",
 					"$(SRCROOT)/../sdk",
 					"$(SRCROOT)/../util",
-					"$(SRCROOT)/../../third-party/nlohmann_json/single_include",
+					"$(SRCROOT)/../../../../../third-party/json/single_include",
 					"$(SRCROOT)/../../third-party/coremltools/deps/protobuf/src",
 				);
 				IPHONEOS_DEPLOYMENT_TARGET = 16.0;
@@ -957,7 +957,7 @@
 					"$(SRCROOT)/../include/executorch/runtime/core/portable_type/c10",
 					"$(SRCROOT)/../sdk",
 					"$(SRCROOT)/../util",
-					"$(SRCROOT)/../../third-party/nlohmann_json/single_include",
+					"$(SRCROOT)/../../../../../third-party/json/single_include",
 					"$(SRCROOT)/../../third-party/coremltools/deps/protobuf/src",
 				);
 				IPHONEOS_DEPLOYMENT_TARGET = 16.0;

backends/apple/coreml/scripts/install_requirements.sh

@@ -49,14 +49,6 @@ mkdir "$COREMLTOOLS_DIR_PATH/build"
 cmake -S "$COREMLTOOLS_DIR_PATH" -B "$COREMLTOOLS_DIR_PATH/build"
 cmake --build "$COREMLTOOLS_DIR_PATH/build" --parallel --target mlmodel
 
-echo "${green}ExecuTorch: Cloning nlohmann."
-git clone https://github.com/nlohmann/json.git "$COREML_DIR_PATH/third-party/nlohmann_json"
-STATUS=$?
-if [ $STATUS -ne 0 ]; then
-    echo "${red}ExecuTorch: Failed to clone nlohmann."
-    exit 1
-fi
-
 echo "${green}ExecuTorch: Copying protobuf files."
 mkdir -p "$COREML_DIR_PATH/runtime/sdk/format/"
 cp -rf "$PROTOBUF_FILES_DIR_PATH" "$COREML_DIR_PATH/runtime/sdk/format/"

third-party/CMakeLists.txt

@@ -4,6 +4,7 @@
 # This source code is licensed under the BSD-style license found in the
 # LICENSE file in the root directory of this source tree.
 
+add_subdirectory(json)
 
 # MARK: - flatbuffers