feature: Add `@IsCUID` validator

[MEDATTA0]

Description

I'm using better-auth in my projects, and it automatically generates CUIDs, by default, for tables that it internally manages.
When I tried to validate these IDs with class-validator, I realized there's no built-in CUID validator. I had to write a custom one manually, copy and paste the same CUID validator across projects.

Proposed solution

I would like to propose adding an @IsCUID() and isCUID validators to class-validator.