AWS credentials chain

What does this MR do?

Adds support for EC2 instance profile credentials when using AWS hosted elasticsearch clusters

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

Could only use static IAM credentials to access AWS hosted elasticsearch cluster

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • [?] All builds are passing - failing on unrelated things
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes #2169 (closed)