Add get_ui_url function

[Y--]

As reported in #35, users would like to have a way to get the UI url.

This PR adds a simple get_ui_url table function such that:

-- Raise an error when server is not started:
D from get_ui_url();
Executor Error:
UI server not started

-- Returns the URL if it is:
D from get_ui_url();
┌────────────────────────┐
│         result         │
│        varchar         │
├────────────────────────┤
│ http://localhost:4213/ │
└────────────────────────┘