fix(iam): add ECS credential discovery with coverage

[rhnvrm]

This PR supersedes #43 and addresses #42.

It preserves the original ECS credential discovery contribution as the first commit, then adds the maintainer follow-up needed to make it mergeable:

• preserve IAMResponse compatibility by restoring exported fields removed in added ecs compatibility #43
• validate ECS credential endpoint HTTP status
• drain and close ECS response bodies
• avoid falling back to IMDS when ECS credentials are explicitly configured but the ECS fetch fails
• add regression coverage for:
  • ECS success path
  • ECS non-200 path
  • IMDS fallback when ECS credentials are not configured

Attribution:

• commit feat(iam): add ECS container credential discovery keeps the original author attribution to Antoine Charbonneau and is based on the implementation from added ecs compatibility #43

Tested:

• go test ./... -run 'TestS3_NewUsingIAM|TestFetchIAMData' -count=1
• full local MinIO-backed suite with:
  • AWS_S3_REGION=us-east-1
  • AWS_S3_ACCESS_KEY=minioadmin
  • AWS_S3_SECRET_KEY=minioadmin
  • AWS_S3_ENDPOINT=http://127.0.0.1:9000
  • AWS_S3_BUCKET=testbucket
  • AWS_ACCESS_KEY_ID=minioadmin
  • AWS_SECRET_ACCESS_KEY=minioadmin
  • AWS_DEFAULT_REGION=us-east-1
  • go test -v ./...