Support "Get Partitions Assigned to Adapter" operation in mock support

[andy-maier]

This will fix the following test failure when testing against the example mocked environment:

TESTINVENTORY=examples/example_hmc_inventory.yaml TESTVAULT=examples/example_hmc_vault.yaml make end2end
. . .
FAILED tests/end2end/test_adapter.py::test_adapter_list_assigned_part[hmc_definition=MOCKED_Z16_DPM] - zhmcclient._exceptions.HTTPError: 404,1: Unknown resource with URI: /api/adapters/osa1/operations/get-partitions-assigned-to-adapter [GET /api/adapters/osa1/operations...