[Feature Request] Add `az role assignment show` command

Currently, there is no `show` command under `az role assignment` command group.

```
> az role assignment -h

Group
az role assignment : Manage role assignments.

Commands:
create : Create a new role assignment for a user, group, or service principal.
delete : Delete role assignments.
list : List role assignments.
list-changelogs : List changelogs for role assignments.
update : Update an existing role assignment for a user, group, or service principal.
```

Since `az role assignment create` is going to support custom assignment name, a corresponding command `az role assignment show` can be introduced to show one exact role assignment.

This can be done during the migration to `azure-mgmt-authorization` Track 2 SDK (https://github.com/Azure/azure-cli/issues/23372).

**Describe the solution you'd like**
```
az role assignment show --id /subscriptions/0b1f6471-1bf0-4dda-aec3-cb9272f09590/providers/Microsoft.Authorization/roleAssignments/4513dda8-fbdc-4f04-8b87-74fa11600aea

az role assignment show --scope /subscriptions/0b1f6471-1bf0-4dda-aec3-cb9272f09590 --name 4513dda8-fbdc-4f04-8b87-74fa11600aea
```