[roll third_party/pigweed] pw_protobuf: Support proto3 optional fields

When a proto3 field is labelled as an optional, wrap the struct Message
type with std::optional<> to support presence.

Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/91904

https://pigweed.googlesource.com/pigweed/pigweed
third_party/pigweed Rolled-Commits: 48df483dbb69d7e..ee7729707ce0551
Roller-URL: https://ci.chromium.org/b/8811712773755409969
Cq-Cl-Tag: roller-builder:pigweed-experimental-roller
Cq-Cl-Tag: roller-bid:8811712773755409969
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: Ib4d4e09b14660b7803dc9367691654796f9cf929
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/experimental/+/97787
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 48df483..ee77297 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit 48df483dbb69d7ecfbe01e7f94832b855758e412
+Subproject commit ee7729707ce0551541a9e1f6152aa8fd246f956e