[1/2] Support rendering differential flamegraphs in the `StackTraceFl…

…ameGraph` display spec (#2043)

Summary: Support rendering differential flamegraphs in the
`StackTraceFlameGraph` display spec

This change makes it possible to render a [differential
flamegraphs](https://www.brendangregg.com/blog/2014-11-09/differential-flame-graphs.html).
These are useful for comparing flamegraph profiles against each other in
order to see what code paths are called more or less than the baseline
profile. When a Vis spec specifies a `differenceColumn`, the widget will
render this differential flamegraph. The difference column computes the
sample delta between the baseline profile and the new profile.

Relevant Issues: N/A

Type of change: /kind feature

Test Plan: New unit tests pass and pxl script relying on this works as
expected (see #2044)


![differential](https://github.com/user-attachments/assets/3f86ee2d-c0a1-402b-9455-b1e22b522fe1)

Changelog Message: Add support for rendering differential flamegraphs in
the `StackTraceFlameGraph` display spec