Add private metrics DataUploadConfig downloader

Add the implementation for PrivateMetricsDataUploadConfigDownloader.
This class is responsible for downloading a serialized protocol buffer
of DataUploadConfig from Gstatic. DataUploadConfig contains the public
key needed to encrypt private metric reports. It also includes signed
endorsements for the keys, which are necessary to perform attestation
verification.

Bug: 433730177
Change-Id: Ie6eec5785ba9702540c928676bc798bf6709eb43
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6873142
Reviewed-by: Alexei Svitkine <asvitkine@chromium.org>
Commit-Queue: Jay Zhou <zhouzj@google.com>
Cr-Commit-Position: refs/heads/main@{#1506707}
NOKEYCHECK=True
GitOrigin-RevId: 9864d0c3841e4e94a421e2b7f2cd1ab18c4563b1