fix: allow IPv6 EKS Pod Identity host regardless of IMDS endpoint mode

[jtuglu1]

ContainerCredentialsProvider rejects the EKS Pod Identity IPv6 endpoint (http://[fd00:ec2::23]/v1/credentials) unless AWS_EC2_METADATA_SERVICE_ENDPOINT_MODE was explicitly set to IPV6. That variable configures the EC2 IMDS endpoint (169.254.169.254 vs [fd00:ec2::254]) which is (I believe?) a separate subsystem from the container credentials endpoint and EKS Pod Identity never sets it, so it defaults to the IPv4 allowlist. This behavior rejects valid IPv6 loopback hosts.

This change aligns the Java SDK with the C++, Python (botocore), Go v2, and JS v3 SDKs, all of which allow the EKS IPv6 host unconditionally.

Related issue: aws/containers-roadmap#2683.

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• New feature (non-breaking change which adds functionality)

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