Support provisioning instances without public IPs on AWS

[r4victor]

Currently, dstack-provisioned instances always have public IP addresses assigned. Users may want to provision instances without public IPs and connect to instances via internal VPC addresses for security reasons.

The proposal is to introduce public_ips: true/false field to the backend config:

type: aws
  creds:
    type: default
  public_ips: false

When public_ips: true, dstack provisions a new instance with a public IP (the current behavior). When public_ips: false, dstack assigns no public IP and uses a private IP to connect to the instance.

Users need to make sure that the dstack server, gateways, and dev machines can access the instances via internal IPs. For example:

• To access instance from dev machines, users set up VPN to the VPC, e.g. AWS Client VPN.
• To connect different VPCs in different regions, VPC-peering can be used.

We'll start by supporting instances without public IPs on AWS. We can then support Azure, GCP, etc.