Conditional Attribute Support

[steve-r-west]

The above is maybe an extreme case, but in other cases like custom-fields, we should just support :

validation.string.min_length:
type: INT
when: field_type == string

Maybe something like this:
https://github.com/expr-lang/expr

We will need to do something more advanced in #572

[steve-r-west]

We can and should also support when on the base attributes.... The above is maybe an extreme case, but in other cases like custom-fields, we should just support

validation.string.min_length:
type: INT
when: field_type == string