[roll third_party/pigweed] pw_blob_store: Make BlobReader seekable

UpdateBundle will need to parse proto on strage using the coming
protobuf::StreamDecoder with BlobStore::BlobReader. However, the decoder
will only accept SeekableReader. The CL makes BlobReader seekable to
accommodate the decoder.

Original-Bug: 456
Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/59420

https://pigweed.googlesource.com/pigweed/pigweed
third_party/pigweed Rolled-Commits: 64021a8b197d3cb..4a6deb42dde7d4d
Roller-URL: https://ci.chromium.org/b/8837240697508341137
Cq-Cl-Tag: roller-builder:pigweed-experimental-roller
Cq-Cl-Tag: roller-bid:8837240697508341137
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: I1780e14bf6c59a0954f2bceb9b88645cb37f1136
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/experimental/+/59443
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 64021a8..4a6deb4 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit 64021a8b197d3cbc5fc244ceb1cfd03004dc4f18
+Subproject commit 4a6deb42dde7d4d8b081bae6c6688d824b960034