Don't use gnmic

Rework gnmi helper method to use the python client instead of a binary that may or may not be installed on the system where the tests are running. Such implementation already exists here and can be copied: https://code.inmanta.com/solutions/modules/connect/-/blob/c502791309edee386b4a987673070c5762dec758/pytest-inmanta-connect/src/pytest_inmanta_connect/helpers/nokia_router.py#L394