Feature: Support .env files for env vars

[maha-hajja]

Feature description

for env vars used in pipeline config file, support providing the .env file to get the env vars from

[timwehrle]

I can look into this.

[lovromazgon]

This can already be done with tools like dotenv. What we need to do though is document the environment variables thoroughly (tracked in this issue: ConduitIO/conduit-site#150).