Bug: User credentials (authorized_user) not working

When using `gcloud auth application-default login`, the proxy fails with:

```
failed to find token source: failed to get idtoken source: idtoken: unsupported credentials type
```

## Cause

The error message from `idtoken.NewTokenSource()` changed in [google.golang.org/api v0.106.0](https://github.com/googleapis/google-api-go-client/commit/f6dec99d014cc508d0374b666f2c325534a30bea):
- Before: `"credential must be service_account"`
- After: `"unsupported credentials type"`

The string check in `findTokenSource()` still looks for the old message, so user credentials are not recognized.

This bug was introduced in #21 when google.golang.org/api was bumped to v0.110.0.
It's strange that this bug has gone unnoticed for so long, but I think this is the cause.