Convert upload client to kotlin

[psh]

Given the centrality and size of the UploadClient this PR was split out the work of converting clients to Kotlin to make sure all is good and verifiable.

Getting good tests around the UploadClient meant some refactoring

• to pull out static references to other classes
• to go down the road of making the Contribution class a little more OO by encapsulating logic that was touching its internal state
• constructing FileUtilsWrapper with a context (provided by Dagger in the real production code) to remove the context from the helper method interface

Once the test was in place around the Java version, it provided safety to convert the Java implementation to Kotlin and verify that none of its behavior had changed.

Note: this relates to the discussion in #4665

[psh]

The functional interface TimeProvider is designed to let us pass a lambda that returns a Long value e.g.

{ System.currentTimeMillis() }

Which then simplifies to the method reference you see here. All this facilitates massing a mock<TimeProvider>() in unit tests giving us control of the time we use in the upload client 😄

https://kt.academy/article/ek-function-types

[nicolas-raoul]

Tried 1 single upload and 2 multi-uploads, seems to work normally.
Thanks a lot! :-)