[roll pigweed] pw_sync: Add ConditionVariable facade

ConditionVariable mimics std::condition_variable in terms of API and
semantics. There is one provided backend in pw_sync_stl.

Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/95001

https://pigweed.googlesource.com/pigweed/pigweed
pigweed Rolled-Commits: c0225ca71894222..9235aeb653e684a
Roller-URL: https://ci.chromium.org/b/8806911665383813681
Cq-Cl-Tag: roller-builder:pigweed-zephyr-roller
Cq-Cl-Tag: roller-bid:8806911665383813681
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: I9fcee99748034bb86f519c6363e0b9a5967e7833
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/zephyr-integration/+/104710
Bot-Commit: Pigweed Integration Roller <pigweed-integration-roller@pigweed.google.com.iam.gserviceaccount.com>
Commit-Queue: Pigweed Integration Roller <pigweed-integration-roller@pigweed.google.com.iam.gserviceaccount.com>
diff --git a/pigweed b/pigweed
index c0225ca..9235aeb 160000
--- a/pigweed
+++ b/pigweed
@@ -1 +1 @@
-Subproject commit c0225ca71894222e6b1f0e1e26d79fe1bf4d3294
+Subproject commit 9235aeb653e684a6f0b7b563965d85c747281a0f