FR: get doc of sub-collections at once

### [REQUIRED] Describe your environment

* Operating System version: JS
* Firebase SDK version: 4.11
* Firebase Product: firestore

### [REQUIRED] Describe the problem
if we have some data like;

- collection-department
- dept1
- collection-employee
- emp2
- emp1
- dept2
- collection-employee
- emp5
- emp8
- dept3
- collection-employee
- emp6
- emp10

- another-collection

if we want the employees in dept1
`db.collection('/collection-department/dept1/collection-employee').get()`

we need to get all employees in all department in one request; something like
`db.collection('/collection-department/*/collection-employee').get()`

my case is efferent but this sample for explanation.. that we cant USE the sub collection feature if we miss this option.. our scenario have a thousand of document that share the same sub-collection and we need to get the data in single request not doing a thousands one...

Thanks