zipped plugins don't work in newer releases

[Luciano Lo Giudice]

The grafana charm seems to not work with .zip plugins. In the ceph-dashboard charm CI tests, the charm is using the following plugin: https://storage.googleapis.com/plugins-community/vonage-status-panel/release/1.0.11/vonage-status-panel-1.0.11.zip,https://storage.googleapis.com/plugins-community/grafana-piechart-panel/release/1.6.2/grafana-piechart-panel-1.6.2.zip

which fails with the following error:

```
2023-03-20 19:50:08.730097 | focal-medium | 2023-03-20 19:50:08 [ERROR] unit-grafana-0.log: Traceback (most recent call last):
2023-03-20 19:50:08.730112 | focal-medium | 2023-03-20 19:50:08 [ERROR] unit-grafana-0.log: File "/var/lib/juju/agents/unit-grafana-0/.venv/lib/python3.8/site-packages/charms/reactive/__init__.py", line 74, in main
2023-03-20 19:50:08.730533 | focal-medium | 2023-03-20 19:50:08 [ERROR] unit-grafana-0.log: bus.dispatch(restricted=restricted_mode)
2023-03-20 19:50:08.730566 | focal-medium | 2023-03-20 19:50:08 [ERROR] unit-grafana-0.log: File "/var/lib/juju/agents/unit-grafana-0/.venv/lib/python3.8/site-packages/charms/reactive/bus.py", line 390, in dispatch
2023-03-20 19:50:08.730584 | focal-medium | 2023-03-20 19:50:08 [ERROR] unit-grafana-0.log: _invoke(other_handlers)
2023-03-20 19:50:08.730597 | focal-medium | 2023-03-20 19:50:08 [ERROR] unit-grafana-0.log: File "/var/lib/juju/agents/unit-grafana-0/.venv/lib/python3.8/site-packages/charms/reactive/bus.py", line 359, in _invoke
2023-03-20 19:50:08.730610 | focal-medium | 2023-03-20 19:50:08 [ERROR] unit-grafana-0.log: handler.invoke()
2023-03-20 19:50:08.730643 | focal-medium | 2023-03-20 19:50:08 [ERROR] unit-grafana-0.log: File "/var/lib/juju/agents/unit-grafana-0/.venv/lib/python3.8/site-packages/charms/reactive/bus.py", line 181, in invoke
2023-03-20 19:50:08.730660 | focal-medium | 2023-03-20 19:50:08 [ERROR] unit-grafana-0.log: self._action(*args)
2023-03-20 19:50:08.730672 | focal-medium | 2023-03-20 19:50:08 [ERROR] unit-grafana-0.log: File "/var/lib/juju/agents/unit-grafana-0/charm/reactive/grafana.py", line 324, in install_plugins
2023-03-20 19:50:08.731134 | focal-medium | 2023-03-20 19:50:08 [ERROR] unit-grafana-0.log: fetch.install_remote(plugin_url, dest=plugins_dir)
2023-03-20 19:50:08.731169 | focal-medium | 2023-03-20 19:50:08 [ERROR] unit-grafana-0.log: File "/var/lib/juju/agents/unit-grafana-0/.venv/lib/python3.8/site-packages/charmhelpers/fetch/__init__.py", line 182, in install_remote
2023-03-20 19:50:08.731187 | focal-medium | 2023-03-20 19:50:08 [ERROR] unit-grafana-0.log: raise UnhandledSource("No handler found for source {}".format(source))
2023-03-20 19:50:08.731201 | focal-medium | 2023-03-20 19:50:08 [ERROR] unit-grafana-0.log: charmhelpers.fetch.UnhandledSource: No handler found for source https://storage.googleapis.com/plugins-community/vonage-status-panel/release/1.0.11/vonage-status-panel-1.0.11.zip
```

The URL exists, but it seems like it being zipped is now an issue (it used to work fine).

I'm attaching a bundle sample we use. For the full logs, see: https://openstack-ci-reports.ubuntu.com/artifacts/27c/877704/6/check/jammy-yoga/27c9942/job-output.txt