feat: adds trust boundary lookup support for SA and impersonated credentials

[aeitzman]

[parthea]

Switching to draft as there are merge conflicts

[chalmerlowe]

Closing:

• OP provided no description or context
• OP added a do not merge label with no context or removal criteria
• No action has occurred in close to 11 months

If this is a change that is deemed critical, we can re-open on the new location once this repo has been migrated to the google-cloud-python mono-repo.