PHP: Create validator function to validate constraints

[spinillos]

The fields set in the constructor don't have any constraint validation and we can put invalid values.

[K-Phoen]

I don't think we should do this (for PHP, but also for the other languages).

The pattern we have in Go seems to work well: a dumb constructor that accepts any value + a Validate() function that can be called by the builders or whenever the user wants.

I've got two main reasons why I'm not in favor of the change described here:

• I view types generated by cog as DTOs (just data transfer objects, with the absolute minimum in terms of logic, including validation)
• for some languages like PHP or Python, validation in the constructor would make it impossible to unmarshal objects from JSON when they violate these validation rules