Support gateways with multiple replicas

[jvstme]

A gateway can now have multiple replicas for
improved availability.

type: gateway
name: example-gateway

backend: aws
region: eu-west-1

domain: example.com

certificate: null
replicas: 2

To balance requests between gateway replicas, add
DNS records for each replica or set up a load
balancer outside of dstack. Replica hostnames
are displayed in dstack CLI and UI.

$ dstack gateway list
 NAME             BACKEND          HOSTNAME        DOMAIN       DEFAULT  STATUS
 example-gateway                                   example.com  ✓        running
    replica=0     aws (eu-west-1)  34.244.128.46
    replica=1     aws (eu-west-1)  18.201.201.174

Limitations:

• Changing the number of replicas or redeploying
  replicas is not supported.
• HTTPS is not supported. Use an external load
  balancer for TLS termination.
• An unavailable gateway replica prevents any new
  services or service replicas from being added.
• All replicas are bound to the same backend and
  region.

Implementation notes:

• GatewayComputeModel now represents a gateway
  replica.
• In this version, the terms "compute" and
  "replica" are used interchangeably. The plan is
  to switch to using exclusively "replica" later.
• In this version, replica provisioning and
  termination are still done in the gateway
  pipeline, for all replicas at once. The plan is
  to introduce gateway replica pipelines later to
  allow for independent replica processing.

#3959