Create batch pools with configuration of user assigned identities

**Is your feature request related to a problem? Please describe.**
It is now possible to create batch pools that use a user assigned managed identity [see Azure Docs](https://docs.microsoft.com/en-us/azure/batch/managed-identity-pools#create-a-batch-pool-with-user-assigned-managed-identities).
This is a very nice feature to authenticate the access from the batch VMs to other azure services. Right now, it is not possible to create such a configured pool via the CLI - only by using the SDK.

**Describe the solution you'd like**
Simple CLI Parameter to configure the user assigned identites that should be used creating the pool, e.g.

````
az batch pool create \
--id mypool \
--image microsoft-azure-batch:ubuntu-server-container:20-04-lts \
--node-agent-sku-id batch.node.ubuntu 20.04 \
--user-assigned-identities 210d0ab6-d4fe-11eb-a1d7-0f7ac90d9a0e,26185632-d4fe-11eb-8845-733c15eeb6f3
````