Add a helper to determine if an exception is from a 304 Not Modified response.

[darronschall]

In practice, Ktor will throw a RedirectResponseException on a 304 Not Modified server result. This small helper lets us avoid having to repeat ourselves with a cast and status check when we're trying to determine what happened in our exception handling:

try {
   val (response, responseHeaders) = exampleService.getCacheData(...)
} catch (e: ServiceException) {
  if (e.isNotModifiedException()) {
    // Ignore, local cache is not stale.
  }
}

NotModified is not one of the Twirp Error code enums, so we're on our own in coming up with a way to handle it. Adding it as an enum makes a little more sense, but this works without creating our own extension of the Twirp spec.