Issue with key case for write operation

[Cubid]

Documentation states:

Secret keys are normalized automatically. The - will be _ and the letters will be converted to upper case (for example a secret with key secret_key and secret-key will become SECRET_KEY).

When the code is just using lower case

func NormalizeKey(key string) string {
	return strings.ToLower(key)
}

I'm fine with this behavior, but for migration purpose to external-secrets, I need to be able to maintain the case of the environment variables.

I offer to use a --preserve-case flag to ignore the automatic case change, as it is done in env, and also be able to match the import command capabilities.

[Cubid]

Associated PR fix documentation and add the -p flag