[ios] Reduce scope of `:*` permissions

[palpatim]

Currently, the core stack grants cognito-identity:* because the test setup code performs a lot of administrative functions during setup & tear down. We should investigate to see if we can reduce scope.

The same can be said for:

• Any test module that invokes add_to_common_role_policies() without any qualifiers (https://github.com/aws-amplify/amplify-ci-support/blob/master/src/integ_test_resources/common/common_stack.py#L43)
• https://github.com/aws-amplify/amplify-ci-support/blob/master/src/integ_test_resources/common/auth_utils.py#L68 (we should also consolidate the assignment of cognito-identity permissions while we're investigating)

It would be feasible to either reduce scope of the actions, or the resources on which those actions may be performed.