[roll third_party/pigweed] pw_software_update: Add UpdateBundle class

Adds an empty UpdateBundle class. It is used by the "Update
Manager" to parse, verify and provide access to a staged firmware
update bundle.

Original-Bug: 456
Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/58043

https://pigweed.googlesource.com/pigweed/pigweed
third_party/pigweed Rolled-Commits: 633d678a5900494..637b773dccf3927
Roller-URL: https://ci.chromium.org/b/8837265533447344017
Cq-Cl-Tag: roller-builder:pigweed-sample-project-roller
Cq-Cl-Tag: roller-bid:8837265533447344017
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: Iebe03b57ea3787684fe8bea7e8a90674db262602
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/sample_project/+/59340
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 633d678..637b773 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit 633d678a5900494d8ffa7cafee84f818c132815a
+Subproject commit 637b773dccf39275a6e9df9bd8dcc4d3598f3ce2