pw_thread: add DetachedThread helper function

Adds the pw::thread::DetachedThread function as a helper to spawn
detached threads, where the user doesn't have to worry about the
thread handle and detaching it.

Change-Id: I179e342d8e0a79a08243a4438176b1249e13c660
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/42864
Reviewed-by: Wyatt Hepler <hepler@google.com>
Commit-Queue: Ewout van Bekkum <ewout@google.com>
diff --git a/pw_thread/BUILD b/pw_thread/BUILD
index 37ab883..c2b67b1 100644
--- a/pw_thread/BUILD
+++ b/pw_thread/BUILD
@@ -92,6 +92,7 @@
     name = "thread_facade",
     hdrs = [
         "public/pw_thread/thread.h",
+        "public/pw_thread/detached_thread.h",
     ],
     includes = ["public"],
     deps = [