[roll third_party/pigweed] pw_protobuf: Generate enum-to-string functions

Generate an <EnumName>ToString() function alongside the enum and the
IsValid<EnumName>() function.

Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/126683

https://pigweed.googlesource.com/pigweed/pigweed
third_party/pigweed Rolled-Commits: 71ba37c8a6cc65c..54d29b131396e04
Roller-URL: https://ci.chromium.org/b/8791146510306921617
GitWatcher: ignore
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: I07575c544d07fda8318adcdb45ec5a7a5a190b6c
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/experimental/+/126811
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 71ba37c..54d29b1 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit 71ba37c8a6cc65c8084078fb000e8f470cff8597
+Subproject commit 54d29b131396e046a882a656c8d96adb5ddad5c7