[roll third_party/pigweed] pw_kvs: Add value_type to Key

Some std::span implementations rely on the value_type for the
containers. This CL adds proper value_type for the Key.

Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/99281

https://pigweed.googlesource.com/pigweed/pigweed
third_party/pigweed Rolled-Commits: 522e55dae4bfbed..39336b04b150402
Roller-URL: https://ci.chromium.org/b/8810532292528166337
Cq-Cl-Tag: roller-builder:pigweed-experimental-roller
Cq-Cl-Tag: roller-bid:8810532292528166337
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: I08688f2a2125f7321302d229416f2709aeafb5e5
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/experimental/+/99526
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 522e55d..39336b0 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit 522e55dae4bfbed7727a1601bef8e94d5d59dc34
+Subproject commit 39336b04b15040237cc298da9756a16945054d57