Provide default implementation of QuicheThread.

This uses STL thread class, which is not allowed inside google3 or Chromium (since those have their own thread abstractions), but is the best we can get in an open-source project otherwise.

PiperOrigin-RevId: 449014471
diff --git a/build/source_list.gni b/build/source_list.gni
index 0246d0d..9f1ee66 100644
--- a/build/source_list.gni
+++ b/build/source_list.gni
@@ -1400,6 +1400,7 @@
 ]
 default_platform_impl_test_support_hdrs = [
     "src/quiche/common/platform/default/quiche_platform_impl/quiche_test_loopback_impl.h",
+    "src/quiche/common/platform/default/quiche_platform_impl/quiche_thread_impl.h",
 ]
 default_platform_impl_test_support_srcs = [
     "src/quiche/common/platform/default/quiche_platform_impl/quiche_test_loopback_impl.cc",