Add IPv6 validator for strings

[rshelekhov]

Hi @Oudwins, following the recent implementation of the IPv4 validator (PR #192), I'd like to propose adding an IPv6 validator to complete the IP validation suite.

Example usage

field := z.String().IPv6()
err := field.Validate("2001:db8::1") // Valid
err = field.Validate("192.168.1.1") // Invalid (IPv4)
err = field.Validate("::ffff:192.168.1.1") // Valid (IPv6-mapped IPv4)

The groundwork is already in place in your lib:

• IssueCodeIP constant exists
• Error message template supports {{ip}} placeholder
• Just need to add IPv6 = "IPv6" constant
• Validator logic would be like this (I checked how it's implemented in go-playground/validator):

ip := net.ParseIP(string(*v));
return ip != nil && ip.To4() == nil

Unlike IPv4 validator which rejects any : syntax, IPv6 validator should:

• Accept pure IPv6: "2001:db8::1"
• Accept IPv6-mapped IPv4: "::ffff:192.168.1.1"
• Reject pure IPv4: "192.168.1.1"

Question for Discussion

Should we also add a generic IP() validator that accepts both IPv4 and IPv6? Perhaps this would be useful for cases where users don't care about the specific version.

If you're open to this addition, I'm happy to implement it following the same patterns we established for the IPv4 validator.

[Oudwins]

Hey @rshelekhov !

Yes! This would be awesome. I think it will go much faster also now that the pattern is in place. Should be less work.

I think adding the IP validator is also worth it.

[Oudwins]

Thank you so much for your work! You rock. Closing this now