roll: third_party/pigweed pw_protobuf: Force use of callbacks for oneof

pw_protobuf's message structures previously generated each of the
members of a oneof group as a separate struct member, allowing multiple
of them to be set to serialize semantically-invalid wire messages.

This replaces oneof struct member generation with a single callback for
the entire oneof group, allowing the user to encode/decode the desired
oneof field using the wire stream encoder and decoder.

The old oneof behavior is kept behind a protoc plugin flag to aid
downsteam migration. This flag is currently only activated in Bazel
builds.

Original-Bug: 373693434
Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/242392
Original-Revision: 205570386eac8fe6e0269b7fbbab1449eb565036

Rolled-Repo: https://pigweed.googlesource.com/pigweed/pigweed
Rolled-Commits: 6ae64ef2889810..205570386eac8f
Roll-Count: 1
Roller-URL: https://ci.chromium.org/b/8733872348965579505
GitWatcher: ignore
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: Iddad339fab4af5b24e612f8f72fb45db5efd82ea
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/experimental/+/242872
Bot-Commit: Pigweed Roller <pigweed-roller@pigweed-service-accounts.iam.gserviceaccount.com>
Commit-Queue: Pigweed Roller <pigweed-roller@pigweed-service-accounts.iam.gserviceaccount.com>
Lint: Lint 🤖 <android-build-ayeaye@system.gserviceaccount.com>
diff --git a/third_party/pigweed b/third_party/pigweed
index 6ae64ef..2055703 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit 6ae64ef2889810d43682b85e0c793018ae9a507c
+Subproject commit 205570386eac8fe6e0269b7fbbab1449eb565036