Add support for Kotlin's Result

Now that inline classes are stable in Kotlin 1.5.0, it would be nice to have API endpoints return [Result](https://kotlinlang.org/api/latest/jvm/stdlib/kotlin/-result/) to wrap the outcome (success/failure) of a retrofit call, instead of returning a value or throwing an exception.

For example:

```
@GET("me")
suspend fun getUser(): Result<User>
```