Support for AWS EleasticSearch

[KyleMasterson]

I am currently attempting to use the exporter to expose metrics from an AWS ElasticSearch instance, but all of my requests are failing with status code 401:

level=warn ts=2020-05-19T18:21:49.238735875Z caller=cluster_health.go:270 msg="failed to fetch and decode cluster health" err="HTTP Request failed with code 401"

Due to my configuration, it seems basic auth is not an option and I would need requests to be signed. Just wondering if this is a supported use case.