This merges the code for the `significant_terms` agg into the package
for the code for the `terms` agg. They are *super* entangled already,
this mostly just admits that to ourselves.
Precondition for the terms work in #56487
This moves the testing of custom significance heuristic plugins from an
`ESIntegTestCase` to an example plugin. This is *much* more "real" and
can be used as an example for anyone that needs to actually build such a
plugin. The old test had testing concerns and the example all jumbled
together.