Map visualization - support currency options in settings

**Is your feature request related to a problem? Please describe.**
The map visualization currently does not provide an option to specify or override the currency unit of the metric displayed on the page.

From customer:
> We have two different Metabase dashboards for our customers, where we use the same questions but then differentiate between Euro and CHF.
> Now I have noticed that I cannot change the € sign on this chart as I can on all other charts. Why is that?

**Describe the solution you'd like**
Allow customizing the currency unit on the visualization settings similarly to other charts.

**Describe alternatives you've considered**
To override the currency unit one could create a model based on the source table, edit the metadata of the model and set the currency unit for the column to be aggregated. Then create a question based on the model using the Map visualization. This is cumbersome and requires introducing or duplicating a model unnecessarily.

**How important is this feature to you?**
Requested by a customer.

**Additional context**
The way this is currently configurable in the Pie viz:

Start by comparing the Map visualization settings with the Pie visualization behavior referenced in the issue. The work is done when Map settings allow the currency unit to be customized or overridden for the displayed metric, without requiring a new model.

Written by the indexing model from the issue text.