Support AWS placement groups for cluster fleets

[r4victor]

Closes #1715

With this PR, instances in AWS fleets with placement: cluster will be grouped in cluster placement groups to provide better connectivity.

Note that placement groups are not used when running multi-node tasks without first creating a cluster fleet. (We'll see if it should be implemented too).

Fleet example:

type: fleet
name: my-cluster-fleet
nodes: 4
placement: cluster
backends: [aws]

The dstack server creates and deletes placement groups transparently for users. A placement group is created when the first fleet instance is provisioned and deleted by a background task after the fleet is deleted and there are no more instances in the group.