Initial build boilerplate for experimental remoting directory. (#4313)

* Changes the existing IREE_BUILD_EXPERIMENTAL option to IREE_BUILD_EXPERIMENTAL_MODEL_BUILDER.
* Adds an IREE_BUILD_EXPERIMENTAL_REMOTING option.
* Adds a liburing dependency (only enabled for now if IREE_BUILD_EXPERIMENTAL_REMOTING).
diff --git a/.gitmodules b/.gitmodules
index c6ec233..f5306b8 100644
--- a/.gitmodules
+++ b/.gitmodules
@@ -50,3 +50,6 @@
 [submodule "third_party/llvm-bazel"]
 	path = third_party/llvm-bazel
 	url = https://github.com/google/llvm-bazel.git
+[submodule "third_party/liburing"]
+	path = third_party/liburing
+	url = https://github.com/axboe/liburing.git