Allow source/target to be configured via env var

### Description

Currently it is not possible to use env vars to customise the source/target of the nodes as the UI only provides a drop-down select of instances.

This means it is not possible to configure the node to use an env var to set this property. This then makes it hard to use in pipeline based deployments where the value needs to be set on a per-env basis.

The UI should be upgrade to allow for free-form text input as an alternative to selecting from a list.

### Requested By

- https://app-eu1.hubspot.com/contacts/26586079/record/0-1/8977201