x-pack/filebeat/input/gcs: allow absent credentials when using GCS with ADC

[efd6]

Proposed commit message

See title.

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have made corresponding change to the default configuration files
• I have added tests that prove my fix is effective or that my feature works
• I have added an entry in CHANGELOG.next.asciidoc or CHANGELOG-developer.next.asciidoc.

Disruptive User Impact

Author's Checklist

• [ ]

How to test this PR locally

Related issues

• Closes Filebeat Google Cloud Storage Input requires JSON or file credentials for authentication which causes issues when using workload identity #39977

Use cases

Screenshots

Logs

[elasticmachine]

Pinging @elastic/security-service-integrations (Team:Security-Service Integrations)

[ShourieG]

LGTM

[mergify]

This pull request is now in conflicts. Could you fix it? 🙏
To fixup this pull request, you can check out it locally. See documentation: https://help.github.com/articles/checking-out-pull-requests-locally/

git fetch upstream
git checkout -b 39977-gcs upstream/39977-gcs
git merge upstream/main
git push upstream 39977-gcs