[rush] Add bridge-cache plugin

[benkeen]

This PR adds a plugin which allows users to run a Rush command, but only populate the cache from a previous run of that action, not actually run the command.

For instance:

rush build --set-cache-only
rush test --only packageX --set-cache-only
rush lint --to packageY --set-cache-only

Each of these would populate the cache for one or more package actions from whatever was found on disk, as defined for the output folder(s) for the commands in rush-project.json. The command itself (build, test, lint) won't get run.

Why?

We're using a different build orchestrator to handle some high-volume builds (BuildXL), primarily because it's able to farm out work across agents, not just processes. As a build orchestrator, it does exactly the same work as what Rush would have done, but has its own cache. We rely on our current (Rush) pipeline populating the remote cache so our local builds are snappy. Without the feature in this PR, we'd need a separate pipeline running in addition to our main pipeline, solely to populate the Rush cache.