This PR changes the two flavors of OIDC authentication mechanisms to
verify the same audience. This allows the same token to pass both
mechanisms. Previously the regular OIDC flavor uses the project id as
its required audience, which does not work for local user credentials
(such as ones used by the nomulus tool), which requires a valid OAuth
client ID as audience when minting the token (project id is NOT a valid
OAuth client ID).
I considered allowing multiple audiences, but the result is not as clean
as just using the same everywhere, because the fall-through logic would
have generated a lot of noises for failed attempts.
This PR also changes the client side to solely use OIDC token whenever
possible, including the proxy, cloud scheduler and cloud tasks. The nomulus
tool still uses OAuth access token by default because it requires USER level
authentication, which in turn requires us to fill the User table with objects
corresponding to the email address of everyone needing access to the tool.
TESTED=verified each client is able to make authenticated calls on QA with or
without IAP.
