Support for skipping ADC/using only `gcloud` auth?

As a developer, when using the plugin in conjunction with `twine` for uploading packages to a registry, I don't typically want to have the `GOOGLE_APPLICATION_CREDENTIALS` set to a local key, since I'm using `gcloud auth login` for most other things. And, developing on a local workstation, I don't have a reachable metadata service either.

In that usage, a typical session looks something like:

```
-> % twine upload --non-interactive --repository-url https://europe-west2-python.pkg.dev/MYPROJECT/py-test-repo/ dist/somepackage-2.0.0* --verbose -c 'hello'
Uploading distributions to https://europe-west2-python.pkg.dev/MYPROJECT/py-test-repo/
dist/somepackage-2.0.0-py3-none-any.whl (261.8 KB)
WARNING:root:Failed to retrieve Application Default Credentials: Could not automatically determine credentials. Please set GOOGLE_APPLICATION_CREDENTIALS or explicitly create credentials and re-run the application. For more information, please see https://cloud.google.com/docs/authentication/getting-started
WARNING:root:Trying to retrieve credentials from gcloud...
username set from keyring
INFO:twine.auth:username set from keyring
WARNING:google.auth.compute_engine._metadata:Compute Engine Metadata server unavailable on attempt 1 of 3. Reason: timed out
WARNING:google.auth.compute_engine._metadata:Compute Engine Metadata server unavailable on attempt 2 of 3. Reason: [Errno 113] No route to host
WARNING:google.auth.compute_engine._metadata:Compute Engine Metadata server unavailable on attempt 3 of 3. Reason: timed out
WARNING:google.auth._default:Authentication failed using Compute Engine authentication due to unavailable metadata server.
WARNING:root:Failed to retrieve Application Default Credentials: Could not automatically determine credentials. Please set GOOGLE_APPLICATION_CREDENTIALS or explicitly create credentials and re-run the application. For more information, please see https://cloud.google.com/docs/authentication/getting-started
WARNING:root:Trying to retrieve credentials from gcloud...
password set from keyring
INFO:twine.auth:password set from keyring
username: oauth2accesstoken
INFO:twine.repository:username: oauth2accesstoken
password:
INFO:twine.repository:password:
Uploading somepackage-2.0.0-py3-none-any.whl
```

Where the ADC error spam and delays checking for a metadata service somewhat hinder the usability of this approach.

If I explicitly set GOOGLE_APPLICATION_CREDENTIALS to something invalid, it appears to skip the metadata check, but still not ideal:

```
> % GOOGLE_APPLICATION_CREDENTIALS=/dev/null twine upload --non-interactive --repository-url https://europe-west2-python.pkg.dev/MYPROJECT/py-test-repo/ dist/somepackage-2.0.0* --verbose -c 'hello'
Uploading distributions to https://europe-west2-python.pkg.dev/MYPROJECT/py-test-repo/
dist/somepackage-2.0.0-py3-none-any.whl (261.8 KB)
dist/somepackage-2.0.0.linux-x86_64.tar.gz (148.6 KB)
dist/somepackage-2.0.0.tar.gz (256.0 KB)
WARNING:root:Failed to retrieve Application Default Credentials: ('File /dev/null is not a valid json file.', JSONDecodeError('Expecting value: line 1 column 1 (char 0)'))
WARNING:root:Trying to retrieve credentials from gcloud...
username set from keyring
INFO:twine.auth:username set from keyring
WARNING:root:Failed to retrieve Application Default Credentials: ('File /dev/null is not a valid json file.', JSONDecodeError('Expecting value: line 1 column 1 (char 0)'))
WARNING:root:Trying to retrieve credentials from gcloud...
password set from keyring
INFO:twine.auth:password set from keyring
username: oauth2accesstoken
INFO:twine.repository:username: oauth2accesstoken
password:
INFO:twine.repository:password:
Uploading somepackage-2.0.0-py3-none-any.whl
```

I'm not sure what the best solution that doesn't compromise usability or debugging for the probably-more-common automation/CI uses, but one idea would be checking for an explicit `GOOGLE_APPLICATION_CREDENTIALS` value and skipping the ADC attempt if it matches (or running it but suppressing output?) I was thinking maybe `/dev/null` as a flag value, since it shouldn't cause any harm to any other processes if they also happen to read it and attempt ADC.

A separate env var (`KEYRING_ARTIFACTREG_AUTH_GCLOUD_FIRST` or something?) could also maybe work.

One final option might be checking if the supplied username is some specific value, but that relies on all the keyring-consuming apps to correctly allow for the keyring interface modifying the provided username to `get_credential()`, which apparently at least Twine doesn't do properly.

See fork/commit referenced below (https://github.com/camresp/artifact-registry-python-tools/tree/poc-skip-adc) for a hacky version of the various options, the 2 env-var variants work, but the username doesn't, at least with `twine`.

Happy to make a better PR if there's interest in one or more of the approaches.