[FEATURE] Configurable HTTP Client

[dennym]

Is your feature request related to a problem? Please describe.
Currently prom_ex is "completely hardcoded" with finch. This makes it impossible to use some other client, e.g. Tesla, hackney, etc
Settings for finch have to be passed through grafana: finch_pools: [] and will be duplicate. Even if there is already an application in the Supervisor I can not pass in MyApp.Finch.

Describe the solution you would like to see
Make http client configurable.

How would you expect this feature to work
config :prom_ex, :http_client, MyApp.HTTPClient

Additional context
Swoosh has some approach to it...
https://github.com/swoosh/swoosh