data-lake: Expose the metric/imperial display setting

### Current behaviour

The setting needs to be manually pulled and processed by individual widgets, and cannot be readily accessed in user-created variables, Actions, or components.

### Expected or desired behaviour

Expose the variable as a string, with a name like `UNITS/displayPreference`, with values of "metric" and "imperial".

This could then be used to facilitate things like #1319, and various user integrations/customisations.

### Prerequisites

- [x] I have checked to make sure that a similar request has not already been filed or fixed.