[roll third_party/pigweed] pw_multisink & pw_ring_buffer: Add Peek APIs

pw_multisink: Add PeekEntry, PopEntry, and CommitEntry APIs.
pw_ring_buffer: Add PeekFrontPreamble to allow a reader peak the user
preamble without the need to copy data. This lets the multisink commit
multiple entries, peeking one at a time to check if it should be popped
without having to get the entry's data.

Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/56246

https://pigweed.googlesource.com/pigweed/pigweed
third_party/pigweed Rolled-Commits: d63bd748ebc9bc0..64021a8b197d3cb
Roller-URL: https://ci.chromium.org/b/8837242163969561249
Cq-Cl-Tag: roller-builder:pigweed-experimental-roller
Cq-Cl-Tag: roller-bid:8837242163969561249
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: Ica6af0b6cb66411a70ddaf80ef0474beabf10dcc
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/experimental/+/59441
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 d63bd74..64021a8 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit d63bd748ebc9bc0cf7cde78b831f8fb9b18840a6
+Subproject commit 64021a8b197d3cbc5fc244ceb1cfd03004dc4f18