Add a new helper class for OAuth operations in start_host

This CL adds a helper which takes an authorization_code and
exchanges it for access/refresh tokens. The purpose is to simplify
the code in start_host so it doesn't need to implement
gaia::GaiaOauthClient::Delegate and track which account the token
is being retrieved for (e.g. user or robot).

This class will be used in a follow-up where I refactor the existing
start_host impls in a base class.

Bug: b/320745119, b/330404222
Change-Id: I9dc49bf1bde1bb406220d3d5efd8c530ff1742d7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5384624


Commit-Queue: Joe Downing <joedow@chromium.org>
Reviewed-by: Lambros Lambrou <lambroslambrou@chromium.org>
Auto-Submit: Joe Downing <joedow@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1277109}