Admin command to list all webhooks

Sometimes it would be useful to see which hooks exist.
For example when cleaning up old hooks and experiments.

I would like to have an admin command for this:

`list webhooks`

```
The following generic webhooks exist:
- „hookname1“ in room #blubb:example.com
- „hookname2“ in room !blubb2:example.com
```

Start by locating the existing admin-command entry point for the `list webhooks` command. Implement the requested listing of generic webhook names and their Matrix rooms, then verify that the command output matches the examples in the issue.

Written by the indexing model from the issue text.