roll: third_party/pigweed pw_async2: Add PW_AWAIT macro

PW_AWAIT is a helper macro similar to PW_TRY_READY_ASSIGN, intended to
simplify blocking on futures.

Before:

  PW_TRY_READ_ASSIGN(auto val, my_val_future_.Pend(cx));
  PW_TRY_READY(my_void_future_.Pend(cx));

After:

  PW_AWAIT(auto val, my_val_future_, cx);
  PW_AWAIT(my_void_future_, cx);

Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/371733
Original-Revision: f5da5b2d79163561c7b3cb80e1228cecf98fbce5

Rolled-Repo: https://pigweed.googlesource.com/pigweed/pigweed
Rolled-Commits: 577355b038cd69..f5da5b2d791635
Roll-Count: 1
Roller-URL: https://cr-buildbucket.appspot.com/build/8688148377919863585
GitWatcher: ignore
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: Ieed6f5573d143b4bf983d2d051372c5ccf66f764
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/examples/+/382392

third_party/pigweed

diff --git a/third_party/pigweed b/third_party/pigweed
index 577355b..f5da5b2 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit 577355b038cd6976ae409562a9f6a21aae0ff529
+Subproject commit f5da5b2d79163561c7b3cb80e1228cecf98fbce5