[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
• Configuring and viewing resource exports and imports in API, CLI, UI
• Evicting jobs if fleet is no longer imported
• Referencing imported fleets in CLI and run configurations using <project>/<fleet> syntax
• (?) Removing fleets from exports on deletion
• (?) Resource export / import events
• Docs