Optionally upload local action's input tree to remote cache

[sluongng]

One of the nice things about having an advanced remote cache implementation is the ability to inspect Action Cache entries and inspect what's going in and out of each action (Spawn).

However, a lot of the time, the interesting part is inside the input root tree of an action. Because the action was executed locally, only the output of the action was uploaded to the remote cache.

It would be nice to provide an opt-in flag enabling input uploads to the Remote Cache for locally executed actions to improve build visibility.