[blinkpy] 2/N: Add BB client with 'GetBuild' and 'Batch' methods

This change introduces a Buildbucket client derived from the common
pRPC implementation. The call to 'SearchBuilds' has been deduped from
'GitCL'. The client also supports new methods 'GetBuild' and 'Batch',
which the 'update-metadata' tool will call to translate explicit build
numbers for CI builders to builds IDs.

Bug: 1299650
Test: ./wpt_upload.py --dry-run
Change-Id: I8bd85b0f6004e79feda990920f73eeb6b7cc5510
Validate-Test-Flakiness: skip
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3817024
Reviewed-by: Weizhong Xia <weizhong@google.com>
Commit-Queue: Jonathan Lee <jonathanjlee@google.com>
Cr-Commit-Position: refs/heads/main@{#1033778}