[FEATURE] Allow flagd debug flag to be set through environment variable

[Kavindu-Dodan]

Background

flagd debug flag is defined at the root level and available as a persistent flag. However persistent flags are not modifiable through environment variables.

Requirement

Given flagd support environment variable-based configurations, we should add support to define debug flag through environment variable (ex:- FLAG_DEBUG or DEBUG)

Also consider the following,

• Given we haven't visited cobra and viper configurations for a long time, it could be a good opportunity to refactor flagd startup setup
• Document FLAGD prefix based environment variable resolving capability

[snosratiershad]

I'm taking this issue