feat(log-ingestor): Support EC2 IAM role authentication using the default AWS SDK credential provider chain (resolves #1939).

[junhaoliao]

Description

Adds support for "type": "ec2" AWS authentication in the Rust components (log-ingestor, api-server, clp-rust-utils), matching the existing Python compression/query-scheduler behaviour. When ec2 auth is selected, the AWS SDK's default credential provider chain is used instead of explicit access key pairs, allowing
the service to run on EC2 instances, ECS/EKS containers, or any environment where credentials are supplied via environment variables, config files, or instance metadata (IMDSv2).

Checklist

• The PR satisfies the contribution guidelines.
• This is a breaking change and that has been indicated in the PR title, OR this isn't a
  breaking change.
• Necessary docs have been updated, OR no docs need to be updated.

Validation performed

validation isn't possible with localstack ec2 cuz its emulated containers don't seem to support metadata iam/security-credentials. to be done with real aws s3 service.

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