feat: add a default validator to make it more convenient

[Linde7777]

This is a draft version, there are tests failed, I will fix them if this idea is passed.
I use init() to initialize the validator, which will cause the validator(atomic.Value) inconsistent, fail the test TestParseWithValidator

[kevwan]

I think you can change your template.

[Linde7777]

A goctl template can do this, but this default validator feature let user don't need to write any code, for the lazy man like me, it is convenient