Have native sigv4 support to be able to use promlens directly with Amazon Managed Service for Prometheus

# Why? #
To simplify the signing process through [AWS Signature Version 4](https://docs.aws.amazon.com/general/latest/gr/signature-version-4.html) 

# Current workaround # 
promlens users need to set up [aws-sigv4-proxy](https://github.com/awslabs/aws-sigv4-proxy) 

# Prior art #
Similar work was done in Grafana: https://github.com/grafana/grafana/pull/27552