[search aggregator] Add OAuth Logic

This CL adds OAuth2 logic to make an authenticated request to get
enterprise suggestions. Note: We still need permissions to use the
defined scopes, so we cannot test whether making the request works.

Bug: 388489672
Change-Id: Id71cc406d2878443cf0e125ca9c88f4eb51219ad
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6169657
Reviewed-by: Moe Ahmadi <mahmadi@chromium.org>
Reviewed-by: manuk hovanesian <manukh@chromium.org>
Commit-Queue: Ananya Seelam <ananyaseelam@google.com>
Cr-Commit-Position: refs/heads/main@{#1408118}