Method hasOnlyElementsOfType for Iterables

Hello everyone,

I was migrating from assertJ to Truth, and I found no alternative method to hasOnlyElementsOfType for Iterables.
I've used as an alternative this method:

```
private void assertHasOnlyElementsOfType(@NonNull final Iterable iterable, @NonNull final Class classType) {
for (Object object : iterable) {
assertThat(object).isInstanceOf(classType);
}
}
```

Could you consider adding something similar in the future?

Thanks in advance,
Keep up the awesome work!