Add QA run aggregate stats API endpoint

[tw4l]

Fixes #1659

Takes an arbitrary set of thresholds for text and screenshot matches as a comma-separated list of floats.

Returns a list of groupings for each that include the lower boundary and count for all sets that have matching data. Would be good if @ikreymer @SuaYoo you can test the return values to see if it's in a form that works for us. Current format of response, with arbitrary made-up data:

{
    "screenshotMatch": [{"lowerBoundary": "0.0", "count": 0},{"lowerBoundary": "0.7", "count": 3}, {"lowerBoundary": "0.9", "count": 5}]
    "textMatch": [{"lowerBoundary": "0.0", "count": 0}, {"lowerBoundary": "0.7", "count": 1}, {"lowerBoundary": "0.9", "count": 2}, {"lowerBoundary": "0.95", "count": 5}]
}

[ikreymer]

I think the output format makes sense, but i think there should always be an entry with {"lowerBoundary": "0.0", right?

[tw4l]

I think the output format makes sense, but i think there should always be an entry with {"lowerBoundary": "0.0", right?

Good call! Updated to manually add any missing thresholds and the implicit 0.0 threshold to the results even when count is 0 within that bucket.

[ikreymer]

Looks good! (And we can figure out what to do for resources in a separate PR)