Missing support for custom enum derives

[kevin-logan]

quick-protobuf provides support for user defined custom struct derives, but they are not applied to enums. This becomes problematic if you're trying to derive something like serde::Deserialize or serde::Serialize, as they will fail unless any enums that are contained in the structs also have those derives.

It should be a relatively quick addition, and I've created a proposed implementation here: #266