[roll third_party/pigweed] pw_package: Add chromium verifier

Add support for downloading chromium verifier source code in "pw
package". It will be used for certificate verification for boringsssl.
Chromium verifier is only a small part of the chromium source. To avoid
downloading the entire gigantic repo, sparse-checkout is used. The
The CL adds support in git_repo.py for sparse checkout and uses it to
selectively checkout sources related to chromium verifier.

Original-Bug: 394
Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/48220

third_party/pigweed Rolled-Commits: b85391953abac61..2f77d1879b2e398
Roller-URL: https://ci.chromium.org/b/8844885830067765120
Cq-Cl-Tag: roller-builder:pigweed-experimental-roller
Cq-Cl-Tag: roller-bid:8844885830067765120
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: I1c99ca49e89fb4763b1a6234034518fcb1213a6a
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/experimental/+/48421
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 b853919..2f77d18 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit b85391953abac61e34d5f3ee40bb9f4d74e8f7f9
+Subproject commit 2f77d1879b2e3987b212a23dd0741e0e21bceccd