Handle configuration by phase

[minkj1992]

There are two phases

1. local
2. prod

To load env variable I picked godotenv

The env variables used for production phase will be managed in the settings of gcp cloud run.

[mcauto]

@minkj1992 Have you ever seen github.com/Netflix/go-env?
It doesn't support .env, but the tag of struct was cool. :)

it's my example - https://github.com/mcauto/todolist-api/blob/main/modules/config/settings.go

[minkj1992]

👏 �Tag feature looks cool, also the go-env' source code is really simple.