chore: add UnionUnmarshaler for responses that are interfaces (#93)

internal/apijson/registry.go

@@ -29,3 +29,13 @@ func RegisterUnion(typ reflect.Type, discriminator string, variants ...UnionVari
 		unionVariants[variant.Type] = typ
 	}
 }
+
+// Useful to wrap a union type to force it to use [apijson.UnmarshalJSON] since you cannot define an
+// UnmarshalJSON function on the interface itself.
+type UnionUnmarshaler[T any] struct {
+	Value T
+}
+
+func (c *UnionUnmarshaler[T]) UnmarshalJSON(buf []byte) error {
+	return UnmarshalRoot(buf, &c.Value)
+}