feat(dashboards): Create HeatMapWidgetVisualization component

[gggritso]

Before having Heat Map support in Explore, one must have a Heat Map component to put into Explore! This is that widget. This is an MVP, it doesn't even have any features aside from plotting that mostly works. What it does have is:

• plotting! It totally plots
• types that are aligned with the new heat map data endpoint that's coming soon
• enough files to work with (a story, some fixtures, etc.)

e.g.,

This component follows a very similar structure to how the time series, and categorical visualization widgets work. They declare a Plottable interface that the visualization component can plot, and each plottable kind (in this case, just HeatMap) works against that interface. There's a decent chance that we'll support other heat map plottables if we want to show percentiles on the same chart.

This is working from @marthalyndon's generous #114616 which spiked this feature a while back. Thank you Martha!

I also had to update BaseChart because it was dropping its visualMap prop on the floor mysteriously!

The BaseChart component is extended with a visualMap prop passthrough since ECharts heat maps require a visual map for color mapping. The HeatMapPlottable interface is intentionally separate from the existing Plottable interface since heat maps have fundamentally different axes (X, Y, Z) compared to time series (X, Y).

Closes LOGS-762

[linear-code]

LOGS-762 Create basic `HeatMapWidgetVisualization` component

[github-actions]

📊 Type Coverage Diff

Metric	Before	After	Delta
Coverage	93.40%	93.40%	±0%
Typed	134,841	134,870	🟢 +29
Untyped	9,532	9,533	🔴 +1

🔍 1 new type safety issue introduced

Non-null assertions (!) (1 new)

File	Line	Detail
static/app/views/dashboards/widgets/heatMapWidget/heatMapWidgetVisualization.tsx	43	plottables.find(plottable => plottable instanceof HeatMap)!

This is informational only and does not block the PR.