feat: auto bind environment variables in viper.Unmarshal

[mathnogueira]

Description

@krakowski did a nice job describing the problem in his PR.

The current viper implementation does not allow viper.Unmarshal environment variables into structs without binding them first (using viper.BindEnv). This behavior results in an "empty" struct (i.e no values filled) if no configuration file is provided with matching keys.

Following @sagikazarmark's suggestion I converted @krakowski's PR to be used inside viper.Unmarshal instead of relying on another function call from the user.

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