Deprioritize GCloudAuthorizedUser

[djc]

While we're not sure this is a regression since #67 we've recently been seeing elevated error rates in production for some of our code running gcp_auth in a Docker container that has the gcloud CLI installed. The error looks like this (this is from a gRPC client call):

Request had invalid authentication credentials. Expected OAuth 2 access token, login cookie or other valid authentication credential. See https://developers.google.com/identity/sign-in/web/devconsole-project.

Code that doesn't have gcloud installed in the container seems to be running without errors.

The documentation for the print-access-token command also lists some limitations:

Note that token itself may not be enough to access some services. If you use the token with curl or similar tools, you may see permission errors similar to "API has not been used in project 32555940559 before or it is disabled.". If it happens, you may need to provide a quota project in the "X-Goog-User-Project" header.

The identity that granted the token must have the serviceusage.services.use permission on the provided project.

We also noted that the order of token acquisition methods tried by the gcp_auth::AuthenticationManager doesn't match what official Google SDKs do -- this PR would bring gcp_auth closer in line with official SDKs.

Because this seems like a substantial change to the documented mechanics, I've included a commit that bumps the version number to the next semver-incompatible release (despite there being no actual changes to the public API). When releasing this change we should publish release notes that clearly point out the change.

(I worked with @valkum on this, hope I'm correctly representing his research.)

[hrvolapeter]

LGTM, I'm just thinking since the order is already being changed if it doesn't make sense to bring the ADC as last step to have the same order as other SDKs

[djc]

Okay, I've aligned the order with the upstream SDKs. Also renamed the ServiceAccount impls a bit to try make the names more descriptive.

[valkum]

Looks good now. Just a single thought. But feel free to merge without following my remarks.

[djc]

@hrvolapeter thanks for the feedback, please have a look to see if you agree with the renaming!

[hrvolapeter]

Looks good and thanks for the changes! 👍

[djc]

Published 0.9.0 on crates.io. I was going to add a GitHub release to write some release notes but I don't think I have permissions to do so.