[roll third_party/pigweed] pw_snapshot: Add snapshot proto

Initial commit for the pw_snapshot module that introduces the proto
format for device snapshots, and documentation that introduces the
pw_snapshot module and how to use it.

Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/38103

third_party/pigweed Rolled-Commits: 1b5d6cb7f005281..179aa8eab01e885
Roller-URL: https://ci.chromium.org/b/8848486380119528752
Cq-Cl-Tag: roller-builder:pigweed-experimental-roller
Cq-Cl-Tag: roller-bid:8848486380119528752
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: I79abacfd1fa79b3a6250caa9cf4794bfff2bc326
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/experimental/+/43163
Reviewed-by: 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 1b5d6cb..179aa8e 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit 1b5d6cb7f00528170e1f3fed3b7493057a0783f3
+Subproject commit 179aa8eab01e885383c1cf730e0e27cafff84f47