Async getUpstream

### Prerequisites

- [x] I have written a descriptive issue title
- [x] I have searched existing issues to ensure the feature has not already been requested

### 🚀 Feature Proposal

The `getUpstream` callback can be sync or async.

### Motivation

We have a (probably unusual) need for `getUpstream` to support async. We have a sophisticated tenancy strategy that requires us to reach out to dbs/caches/etc to resolve the correct upstream.

### Example

```
reply.from(/some/path', {
getUpstream: async () => { return await someAsyncFunc() }
}
```