Look, I don't enjoy having 600 LOC long build.gradle files, it's just
very easy to do! This at least moves some of the complexity elsewhere,
so the build script is a little more declarative.
It's more verbose as the default license plugin doesn't support multiple
license headers. However, it also gives us some other goodies (namely
formatting Kotlin and removing unused imports), so worth doing.
This is a little odd (it's more complex for one!), but means we can
reuse the internal API interface in other classes, which is useful for
the data provider refactor I'm about to do.
This is much nicer in Java 17 :D (records, ServiceLoader.stream()),
but such is the perils of still targetting 1.16.
We now fuzz UploadFileMessage, generating random files and checking they
round-trip correctly.
The joy of having a long-lasting refactor branch with an absolutely
massive diff, is that you end up spotting bugs, and then it's a massive
pain to merge the fix back into trunk!
