Align Get* and Unmarshal behavior better

[sagikazarmark]

Preflight Checklist

• I have searched the issue tracker for an issue that matches the one I want to file, without success.

Problem Description

Different forms of reading from Viper often lead to different results due to different implementations.

For example:

• StringSlice behavior different for command line flag and environment variable #380

Proposed Solution

Use a single implementation (mapstructure+decode hooks? UnmarshalKey?) for all forms of reading from Viper.

Chances are this would be a breaking change, so it either needs a feature flag or needs to be delayed to v2.

Alternatives Considered

No response

Additional Information

No response