Add shim_et nlohmann-json shim for OSS buck2 build

The tokenizers submodule bump (#20472) moved the
fbsource//third-party/nlohmann-json dependency onto the tokenizers :headers
target, which is reachable from the OSS buck2 CI build (multimodal_runner_lib
-> text_token_generator -> tokenizers:headers). Since fbsource aliases to
shim_et, this requires a shim_et//third-party/nlohmann-json package, which did
not exist (only re2 had one).

Add the shim, mirroring shim_et//third-party/re2, aliasing nlohmann-json to the
header-only nlohmann_json target added to the tokenizers submodule
(meta-pytorch/tokenizers#201), and bump the submodule pin to include it.

Co-authored-by: Claude <noreply@anthropic.com>

Author: digantdesai

extension/llm/tokenizers

@@ -0,0 +1,3 @@
+load(":targets.bzl", "define_common_targets")
+
+define_common_targets()

shim_et/third-party/nlohmann-json/BUCK

@@ -0,0 +1,9 @@
+load("@fbsource//xplat/executorch/build:runtime_wrapper.bzl", "runtime")
+
+def define_common_targets():
+    if runtime.is_oss:
+        native.cxx_library(
+            name = "nlohmann-json",
+            exported_deps = ["root//extension/llm/tokenizers/third-party:nlohmann_json"],
+            visibility = ["PUBLIC"],
+        )