Allow multiple pools for one task

Hello! Description of feature: I think it would be helpful to allow for multiple pools in one task. Currently, the `pool` argument for any class inheriting from `BaseOperator` is of type `string`, and thus only allows one pool to be entered. I believe it would be useful to allow to set multiple pools for one task, meaning allowing the argument `pool` to be a `list` of `string` instead of just one `string`. This would mean that a task would have to wait on a spot to be available in every one of the pools it declares, instead of in the only one pool it declares, and this would mean that a task would take up spots in every one of the pools it declares, instead of in only the one pool it declares. Use case: I have some tasks that require multiple resources. I cannot split the tasks into separate tasks each requiring one resource, since the tasks need the two (or more) resources at once to complete their assignment. I also have some tasks only requiring one of the resources, so I can't create a pool for both resources. Example: Task 1 requires resource A and B Task 2 requires resource A Task 3 requires resource B Resource A can only have 4 connections. Resource B can only have 16 connections. I would need to have task 1 be in pool A and pool B, and this is not possible today since I can only specify one pool. What would I want to happen? Allow multiple pools in task creation. I looked into airflow source code, and it looks like the assumption that we only have one pool is deep into SQL, so I cannot just easily fork airflow and add this feature, so the change is not small and I do not have enough airflow understanding to make this change.