Allow a next-stage job read-only access to the cache of a previous-stage one; or make exported/imported artifacts

[native-api]

Introducing Workspaces

We are working on workspaces, a new way of sharing files within a build. Workspaces allow jobs within a build to share files. They are useful when you want to use build artifacts from a previous job; for example, you create a cache that can be used in multiple jobs later. Example configurations and further information can be found in https://docs.travis-ci.com/user/using-workspaces/. We very much appreciate your feedback here. Thank you!