[roll third_party/pigweed] pw_software_update: Use a user_manifest target file

Instead of embedding user metadata in the TargetsMetadata, a
reserved "user_manifest" target file is used to pass the user's
metadata as an opaque blob.

During verification this blob is handed to the backend to verify
when desired.

Original-Requires: pigweed-internal:16500
Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/65361

https://pigweed.googlesource.com/pigweed/pigweed
third_party/pigweed Rolled-Commits: 4cafcda09f52387..12ea8d02bc538d1
Roller-URL: https://ci.chromium.org/b/8833456138024291489
Cq-Cl-Tag: roller-builder:pigweed-experimental-roller
Cq-Cl-Tag: roller-bid:8833456138024291489
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: I6386dc4cab26ecbb8d1d4f11ac7299259de15233
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/experimental/+/65520
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 4cafcda..12ea8d0 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit 4cafcda09f52387ee03bc37743ad5e5168789b25
+Subproject commit 12ea8d02bc538d1e94b3f0788192f33380ccac55