Support explicit Gold service accounts

Supports specifying an explicit service account for Skia Gold to use
instead of relying on LUCI_CONTEXT or gsutil and enables this
functionality for GPU tests running in Skylab. This is necessary
since these tests are automated, which means we can't/shouldn't use
gsutil, but Skylab does not currently support LUCI_CONTEXT.

Bug: 1399908
Change-Id: I43be7f566a1c12c4bbd0efad460c64ebffed7c83
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4093439
Auto-Submit: Brian Sheedy <bsheedy@chromium.org>
Reviewed-by: Sven Zheng <svenzheng@chromium.org>
Reviewed-by: Yuly Novikov <ynovikov@chromium.org>
Commit-Queue: Yuly Novikov <ynovikov@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1081972}