Retrieve an Instance Identity token when configuring Cloud hosts

This CL updates the Cloud host provisioning path to request an Instance
Identity token which will be used for configuring remote access for a
specific Compute Engine Instance (and the policies associated with it),
rather than relying on the configuration set at the Project level.

Bug: 388886766
Change-Id: If796e7a88f862e68412690fbff990f36f4aa7e69
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6270168
Reviewed-by: Gary Kacmarcik <garykac@chromium.org>
Commit-Queue: Joe Downing <joedow@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1420691}