[roll third_party/pigweed] pw_varint: MaxValueInBytes function

This adds a function to the varint API that returns the maximum value
that can be encoded into a given number of bytes.

Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/39581

third_party/pigweed Rolled-Commits: b7902c03b282855..6fe8ccf0ff7e3a1
Roller-URL: https://ci.chromium.org/b/8851102385933124080
Cq-Cl-Tag: roller-builder:pigweed-experimental-roller
Cq-Cl-Tag: roller-bid:8851102385933124080
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: I20cb00fb31b7d59a6c0f8345bffd59074c48a826
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/experimental/+/39765
Reviewed-by: 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 b7902c0..6fe8ccf 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit b7902c03b282855a2fba07c16871097145d7a8af
+Subproject commit 6fe8ccf0ff7e3a13ca4abfdd7c6c452b4d7b8f60