plugin manager: new `timeout` option

When testing plugin manager with real downloads, it could happen that the test run forever. Fortunately, test suite will be interrupted after 20 minutes, but it could be useful not to fail the whole test suite but only warn in that case.

By default, plugin manager will now wait for 5 minutes (300s) to download the plugin from internet. For very big plugins or when the internet connectivity is poor, users can use new `--timeout` option to change this default setting.

```sh
# Will wait for one hour before failing
bin/plugin --install elasticsearch/kibana --timeout 3600

# Will wait indefinitely
bin/plugin --install elasticsearch/kibana --timeout 0
```