Add support for generic structs

[mikemorris]

It would be nice for mog to support conversions to/from types using generic struct fields, such as the following example.

type Gateway struct {
  Status Status[GatewayConditionType]
}

type Route struct {
  Status Status[RouteConditionType]
}

type Status[T ConditionType] struct {
  Type T
}

type ConditionType interface {
  GatewayConditionType | RouteConditionType
}

type GatewayConditionType string
const {
  GatewayConditionAccepted GatewayConditionType = "Accepted"
  GatewayConditionInSync GatewayConditionType = "InSync"
}

type RouteConditionType string
const {
  RouteConditionAccepted RouteConditionType = "Accepted"
  RouteConditionResolvedRefs RouteConditionType = "ResolvedRefs"
}

https://github.com/golang/exp/tree/master/typeparams/example and https://pkg.go.dev/go/types should provide some guidance for how to approach adding this functionality.