Support service scaling via in-place updates

[r4victor]

Closes #1926
Closes #1762

This PR allows updating services replicas and scaling parameters without re-creating the runs, which allows users to scale services manually:

✗ dstack apply -f .dstack/confs/service.yaml
 Project                main                           
 User                   admin                          

...

Active run service4 already exists. Detected configuration changes that can be updated
in-place: ['replicas']
Update the run? [y/n]: y

PR summary:

• Add /api/project/{project_name}/runs/apply endpoint that implements server-side apply (creates or updates a run given run plan input).
• Improve dstack apply CLI command to show different plan messages depending on apply action (create/update/override/etc).
• Add process_runs logic that scales services if the run_spec replicas/scaling parameters change.

Next steps:

• Support services redeployment via rolling deployments. This will allow updating any services parameters and deploy new services versions with zero downtime.