Implement run priorities

[r4victor]

Closes #2564

The PR adds a priority property to run configurations that allows controlling the order in which the runs are provisioned. Runs with higher priorities take precedence over runs with lower priorities. Previously, submitted jobs were processed in FIFO manner with older jobs processed first. Now, the jobs are first sorted by descending priority.

The priority property is updatable, so it can be changed for already submitted runs and will take effect:

Active run test-priority already exists. Detected configuration changes that can be 
updated in-place: ['priority']

Note that if a high priority run cannot be scheduled, it does NOT block other runs with lower priority from scheduling (aka Best effort FIFO). So dstack will try a higher priority run, and if it fails, moves to a lower priority one. This can lead to a lower priority run "overtaking" higher priority run and is considered ok. An alternative is Strict FIFO which blocks lower priority runs until a higher priority run is scheduled. It would be a worse default since one unschedulable run can block all runs in the project, but we may consider supporting it in the future if requested.