feat: add auto credential option for cloud backends

[thweetkomputer]

Auto Credentials Support

• Added cloud_auto_credentials flag to KvOptions, INI parsing, equality, and C/Rust bindings; new APIs let configs skip static cloud_access_key/cloud_secret_key.
• AWS/GCS backend now selects DefaultAWSCredentialsProviderChain when the flag is enabled (with TODO for GCP-native sources) while tests mimic the same behavior; otherwise we
  keep the existing simple provider path.
• Tests gain a provider member to hold the shared credential chain; defaults stay unchanged, so no IMDS call occurs unless the flag is enabled.
• Work lives on branch feature/cloud-auto-credentials in commit Add auto credential option for cloud backends.

[coderabbitai]

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✨ Finishing touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment
• Commit unit tests in branch feature/cloud-auto-credentials

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[github-actions]

[cpplint] _{reported by reviewdog 🐶}
Lines should be <= 80 characters long [whitespace/line_length] [2]

[github-actions]

[cpplint] _{reported by reviewdog 🐶}
Missing username in TODO; it should look like "// TODO(my_username): Stuff." [readability/todo] [2]