Country Map: Add support for labels inside state/province polygons

[bsenthilbalu]

Description
The Country Map visualization in Superset 6 currently supports only hover tooltips (which appear centered at the top of the chart). Because the tooltip does not follow the cursor and cannot be styled, values are often unreadable—especially on US state maps with strong colors.

Requested Feature:
Add support for rendering labels inside the polygon, similar to other mapping libraries (e.g., ECharts, Mapbox).
Requested capabilities:

• Toggle: Show Labels
• Label content options:

-- Region name (ISO‑3166‑2)
-- Metric value
-- region: value combined format

• Label color (light/dark)

• Label font size

• Label position = polygon centroid

Why this is needed:

Country Map is now the primary built-in map in Superset 6.
Without labels, dashboards showing state-level metrics (e.g., packages, revenue) are difficult to interpret.
Hover tooltip is fixed-position, making it unreadable for maps with many small regions.
Many BI tools offer polygon-label rendering as a standard feature.

Version:
Apache Superset 6.x
Screenshot: