feat(go): Unions include runtime validation

[amckinney]

Add runtime validation for discriminated unions to prevent users from accidentally sending the wrong type of value. With this, users will be expected to set exactly one of the union's values like so:

package example

type Animal struct {
  Type string
  Cat  *Cat
  Dog  *Dog
}

func do() {
  union := &Animal{
    Cat: &Cat{
      Name: "Fluffy",
    },
  }
}

If the user sets both Cat and Dog, the user will receive an error when the type is serialized to JSON (i.e. in the json.Marshaler implementation).

[dsinghvi]

🚀