[content-service] ability to exclude content from the backup of `/workspace`

Is your feature request related to a problem? Please describe

Sometimes users add large files to workspaces at runtime, use them, and then delete them, to avoid backing up the files to Gitpod.

Keeping the backup size smaller helps keep workspace restarts fast, because then we do not have to download and untar unnecessary files from object storage.

Describe the behaviour you'd like

Provide a means to exclude file patterns from the backup of /workspace

Describe alternatives you've considered

• Add the files to your .gitignore and delete them before stopping your workspace
• Implement a pre-stop task, and delete files as part of that task

Additional context

leeway run dev:preview persists 20GB to the file system, but, we do not want to back it up, as an example

Start by tracing the content-service backup path for /workspace and how workspace data is persisted to object storage; the issue names no files or tests. Done means users can provide file patterns that are excluded from the backup, with workspace restore behavior and backup size reflecting those exclusions.