Support ECS instance profile IAM task credentials

To support addding ECS instance profile service to elastic search client in GitLab -- gitlab!24926 (closed) -- a change is also called for the indexer to support ECS task role credentials. From a quick look, it appears v1 and v2 Go SDK support this.

Currently, the Go indexer only supports static credentials and EC2 instance profiles. We're hoping to add ECS endpoint credentials to the chain, before the EC2 provider.

Resources:

• gitlab!24926 (closed)
• AWS Go SDK docs
• Possibly relevant code area in the SDK