I/O: Importing from `postgresql://` not implemented?

## Problem
Why is PostgreSQL support not implemented / enabled?
```shell
ctk load table \
  "postgresql://postgres:postgres@postgresql/public?table=demo" \
  --cluster-url="crate://crate:crate@localhost:4200/doc/testdrive"
```
```python
NotImplementedError: Importing resource not implemented yet: postgresql://postgres:postgres@postgresql/public?table=demo
```
