Raster stats

[noisyneuron]

Add raster statistics methods

Raster layers hold bands rather than attributes, so the existing statistics
methods can't answer anything about them. Four new methods summarize one band
of a raster layer over the whole raster, a bounding box, a polygon, or a line:
a set of statistics, a histogram, counts per distinct value, and values sampled
along a line. Bands on a raster layer now carry the identifier those methods
take, plus a display name.

Ground area is exposed rather than a pixel count, since the pixel count depends
on how far the read was downsampled and the product already calls that number
"Area". Nothing here exposes how the read was budgeted, which stays internal
until the product decides to show users which numbers are approximate.

Handlers are in felt/felt#21889.

Test plan

1. With this branch linked into the Felt app, open a map with a raster layer and
   run felt.getRasterAggregates({ layerId, band: "band:1", aggregation: { methods: ["min", "max", "area"] } }).
   Observe a value for each requested statistic.
2. Run the same call with boundary set to a GeoJSON LineString. Observe that it
   throws, naming area as unanswerable for a line boundary.

[noisyneuron]

• Raster stats #140 👈 (View in Graphite)
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[sullvn]

Looks great ✅