[FEATURE] Support Environment Variables

[beeme1mr]

Requirements

Overview

FlagD supports multiple options that affect how consumers interact with it. Options like what port or service provider is being used needs to be in sync with an OpenFeature provider in order to provide a seamless experience.

Criteria

• [x] Define all the options that require environment variables
• [x] Add the defined environment variables to flagD
• [x] Include the environment variable names in the help message
• [x] Include documentation for what environment variables are supported.

[beeme1mr]

This may be possible with Viper if a prefix is set.