Support AWS placement groups #1715

EC2 instances can be launched into a placement group to influence their placement. In dstack, we're particularly interested in supporting Cluster placement groups for cluster fleets and multi-node tasks:

Cluster – Packs instances close together inside an Availability Zone. This strategy enables workloads to achieve the low-latency network performance necessary for tightly-coupled node-to-node communication that is typical of high-performance computing (HPC) applications.

The management of placement groups can be done transparently for users.