Allow end-tasks, which will run when a workspace stops (timeout)

Similar to start-tasks I propose adding end-tasks.
End-tasks will get triggered and run when a workspace gets stopped (timeout)

Why it's necessary:

• In a "regular" (non-gitpod) setup on a local machine, working on projects that store content and changes in a database is fairly straight forward. I can pull the database once, switch between different branches, make changes in the database, and continue working on the project for many days.
• In a Gitpod setup, I can pull the database once, and any changes I make in the database will persist through timeout and a restart of a workspace. The next day (or minute) I want to work on a different branch so I start a NEW workspace, and poof! the database is gone.

How it works / Why it's a good idea:

• Once end-tasks are available, I can make sure that a machine that is about to be stopped, can run a command that will store the important information / database / docker-image in a safe place. And in the next time I open a new workspace, I can choose fetching that database I saved.
• I am sure there are many more scenarios possible that will be helpful for people once end-tasks feature is available.

The issue names the start-tasks documentation as the closest reference; read it first, then trace the workspace stop and timeout lifecycle. Done means a configured end-tasks command runs when a workspace stops due to timeout and can support saving state for a later workspace.