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.

Bug: 394
Change-Id: I7c6695df54727fe204274467956f22cd238033b9
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/48220
Reviewed-by: Rob Mohr <mohrr@google.com>
Reviewed-by: Ali Zhang <alizhang@google.com>
Commit-Queue: Yecheng Zhao <zyecheng@google.com>