[FR] Let specify function region: `getFunctions(undefined, 'southamerica-east1')`

**Is your feature request related to a problem? Please describe.**
Not applicable.

**Describe the solution you'd like**
I need to specify the region of the function that I'm calling when enqueuing a task with `firebase-admin`:
```js
await getFunctions().taskQueue('foo').enqueue({
id: '123',
})
```

**Describe alternatives you've considered**
None.

EDIT: Just found that it's possible to specify the region like the following snippet, but maybe still consider adding the region parameter to `getFunctions()` to make it similar to the JS SDK (as also suggested [here](https://github.com/firebase/firebase-admin-node/issues/1704#issuecomment-1127970126)) or at least mention this in the docs:
```js
await getFunctions().taskQueue('locations/southamerica-east1/functions/foo').enqueue({
id: '123',
})
```

**Additional context**
Firebase JS SDK works that way. You can pass the region as the second argument to `getFunctions()`: https://firebase.google.com/docs/reference/js/functions.md#getfunctions