fix(notifications): contextual override for WebSocket notification URL

[andrewazores]

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Related to #1542

Description of the change:

See #1542. This is a similar change, but for determining the WebSocket server URL rather than the HTTP API base.

Motivation for the change:

Working toward console plugin being able to establish a WebSocket notification connection through the plugin proxy.

How to manually test:

1. Run CRYOSTAT_IMAGE=quay.io... sh smoketest.sh...
2. ...