Automate gauntlet with GitHub actions
Automate gauntlet with GitHub actions

Creates a github workflow that:
- runs on push or PR to master
- tests across a linux, windows, mac matrix
- downloads and caches a suite of JDKs from
  downloads.imagej.net/test/gauntlet
- runs the gauntlet for each platform
- fails the job if any JDKs on any platforms fail

Note that the GitHub actions cache has a 5GB limit per repository, so we
have excluded 32-bit JDKs and all oracle JDKs.

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