Hadrian: Implement cloud build artifact caching

Shake supports cloud cache builds with it's --share option which caches build artifacts. Hadrian already exposes this feature, but it is unclear if it will produce correct results. Much work has already been done, but the expected benefit decreased as the cost continued to rise. Hence this feature has been put on hold.

If you are thinking about picking up this issue:

1. Read developing hadrian. It outlines what is needed to achieve correctness, and the lessons learned the first time around.
2. See #16400 Most remaining work is centred around fixing fsatrace lint errors. That issue roughly summarizes the remaining issues and explains how to collect such results yourself.
3. Fix #16956 to support caching independent of build and source directory location (i.e. relocatable build artifacts).

Feel free to contact me, @DavidEichmann, with any questions.