Support sigv4 to push metric to AWS managed prometheus

[boskiv]

Feature request: Add support for sigv4 method to write to a remote AWS managed prometheus instance

https://prometheus.io/docs/prometheus/latest/configuration/configuration/#remote_write

[niroz89]

PR grafana/xk6-output-prometheus-remote#169 covers the AWS Access Keys credential provider. It would be nice to cover other credential providers especially Assume role credential provider , so we can avoid hardcoding AWS access keys/secret and generate temporary token instead when running k6-oss in AWS environment.