[Rule-based type customization] Support for enum rules

[bjaglin]

Given the current generated code for proto enums and the lack of ScalaPB options to customize them, it's impossible to generically encode PGV enum rules as compile-time constraints.

1. (validate.rules).enum.defined_only
2. (validate.rules).enum.const
3. (validate.rules).enum.in / (validate.rules).enum.not_in

(1) would be fairly simple to implement via something similar to #62

(2) is complex as it would require a generic solution for capturing constants (and pass them as string litterals?)

(3) has the same challenges as (2) and would require a generalized version of (1)