[Feature]: Cross-project SSH fleet sharing

[jvstme]

Problem

It is not possible to share SSH instances across multiple dstack projects.

Solution

https://docs.google.com/document/d/1kU4eQDXZz9ovQ-1V6etrV6wSuIhm-wC9cwqz6hUgeCQ

Implementation plan

• Submitting jobs to imported fleets
• Viewing imported fleets in API, CLI, UI
• Filtering events by imported fleets and instances
• Exports API & CLI
• Imports API & CLI
• Exports & Imports UI
• Evicting jobs if fleet is no longer imported
• Referencing imported fleets in CLI and run configurations via <project>/<fleet>
• Removing fleets from exports on deletion
• (?) Removing exports / imports on project deletion
• (?) Export / import events
• Docs