Add a unified/canonical URI scheme for cluster connections

e.g.:

```
static:///?key=clusterkey&env=live&instid=uuid&partition=gold`
pgsql://user:pass@host/db_name/?key=clusterkey&env=live&instid=uuid&partition=gold`
```

Tracked as [RESDATA-1272](https://jira.dev.bbc.co.uk/browse/RESDATA-1272)