Support custom config key generator

[cxy1cxy2cxy3]

Preflight Checklist

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

Problem Description

viper always assumes a lower-cased key given.
acutally
users need upper-cased or more complex format of key to store/load

Proposed Solution

type Viper struct {
keyGenerator KeyGenerator
}

type KeyGenerator interface{
GetKey(key string) string
}

func (v *Viper) CustomKeyGenerator(kg KeyGenerator) {
v.keyGenerator = kg
}

func (v *Viper) GetKey(key string) {
if v.keyGenerator != nil {
return v.keyGenerator.GetKey(key)
}
return strings.ToLower(key)
}

Alternatives Considered

No response

Additional Information

No response

[github-actions]

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[sagikazarmark]

Viper is lower case for a reason and that's not going to change anytime soon.

See #1014 for more details.