Allow reverting slice lowercasing to the previous behavior

[sagikazarmark]

Preflight Checklist

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

Problem Description

#1387 fixed a bug around how case insensitivity works in Viper. Unfortunately it causes issues for a number of key players in the field.

Proposed Solution

Allow reverting back to the previous behavior using a go tag. This is going to be a Viper v1 thing only.

Viper v2 will either be completely case insensitive or will allow partial case sensitivity.

Alternatives Considered

No response

Additional Information

No response