Allow proc and symbol as values for `only_integer` of `NumericalityValidator`

We have to dynamically decide of the value should be validated as float or as integer. This PR adds the possibility of defining a symbol or a Proc as value for the `only_integer` option, while preserving the old behaviour of just setting a boolean.

I see one difference in behaviour to the existing version: If you provided a symbol instead of a boolean before, it was always handled as truthness (the same as `only_integer: true`). With this PR applied it would raise an `NoMethodError`, because it tries to call the method on the object. Should I handle that to keep full backwards compatibility?

Thank you Rails team for your hard work! Happy to get any feedback :)