[roll third_party/pigweed] pw_sync: add Borrowable helper for transactional guarding

Adds a templated pw::sync::Borrowable wrapper which permits
objects or pointers/references to them to be wrapped with a lock
to enable threadsafety wrapping for data and/or objects with
transactional APIs where an internal lock may not make sense.

Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/59120

https://pigweed.googlesource.com/pigweed/pigweed
third_party/pigweed Rolled-Commits: 88b0f9b0332f07b..f6c3a78788e7cb8
Roller-URL: https://ci.chromium.org/b/8836533318057800433
Cq-Cl-Tag: roller-builder:pigweed-experimental-roller
Cq-Cl-Tag: roller-bid:8836533318057800433
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: Ic57e96acb3a5eaf09162f11ed4a8e77a2a3f2347
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/experimental/+/60342
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/third_party/pigweed b/third_party/pigweed
index 88b0f9b..f6c3a78 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit 88b0f9b0332f07bcc53d01520ba25c8ef5d58d29
+Subproject commit f6c3a78788e7cb8fe094f4042e9e26a7559610bc