[@vcd/ext-cli] Auto-generated plugin should not have memory leaks

[irahov]

Describe the bug
The autogenerated plugin does not unsubscribe from an observable subscription

To Reproduce
Steps to reproduce the behavior:

1. Create a new plugin vcd-ext new ...
2. Take a loook at the autogenerated code:
   this.client.query(Query.Builder.ofType("organization").links(false)).subscribe
   There is no unsubscribe statement.
   Although this is a demo code and most probably it is going to be replaced by the plugin writer it is not goot to suggest code with memory leaks.
   There should be either unsubscribe or pipe(take(1)) statement