Implement fleet apply plan

[r4victor]

Closes #1472

This PR adds a fleet apply plan similar to the run plan:

✗ dstack apply -f .dstack/confs/fleet.yaml
 Project        main                           
 User           admin                          
 Configuration  .dstack/confs/fleet.yaml       
 Type           fleet                          
 Fleet type     cloud                          
 Nodes          2                              
 Placement      cluster                        
 Backends       aws                            
 Resources      2..xCPU, 8GB.., 100GB.. (disk) 
 Spot policy    on-demand                      

 #  BACKEND  REGION        INSTANCE   RESOURCES                   SPOT  PRICE    
 1  aws      eu-west-1     m5.large   2xCPU, 8GB, 100.0GB (disk)  no    $0.107   
 2  aws      eu-central-1  m5.large   2xCPU, 8GB, 100.0GB (disk)  no    $0.115   
 3  aws      eu-west-1     c5.xlarge  4xCPU, 8GB, 100.0GB (disk)  no    $0.192   
    ...                                                                          
 Shown 3 of 82 offers, $40.9447 max

Fleet my-cluster-fleet does not exist yet.
Create the fleet? [y/n]: 

For older servers (<=0.18.16), the plan is shown without offers (since they don't implement the necessary /get_plan API).

It also implements "attach" for fleet apply so that dstack apply does not exit until the fleet is provisioned with the fleet table shown in watch mode. (addresses #1571).