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 still wait indefinitely but it can be modified using new `--timeout` option:

```sh
bin/plugin --install elasticsearch/kibana --timeout 30s

bin/plugin --install elasticsearch/kibana --timeout 1h
```