Add auth parameter to support SigV4

[ugurzongur]

Is your feature request related to a problem? Please describe.
Amazon Managed Service for Prometheus (APS) cannot be queried. This is because APS uses SigV4 but prometheus-api-client-python doesn't support that auth method.

Describe the solution you'd like
Add auth parameter to PrometheusConnect class init so that we can provide SigV4 auth and pass it to prometheus queries

Describe alternatives you've considered
I considered using the headers but SigV4 depends on the payload and date which makes it impossible to achieve with static headers. Even if we make headers changeable for each request we don't have access to payload outsice PrometheusConnect class.

Additional context
N/A