Support gateways behind ALB with ACM certificate

[r4victor]

Closes #1257

This PR:

• Adds certificate field to gateway configuration. Users can specify lets-encrypt for automatic certificates, acm for AWS Certificate Manager certificates, or None for gateways without certificates that will run only http services.
• Add support for provisioning AWS gateways behind ALB to support ACM certificates. Users will point the domain to the ALB DNS name – the gateways now have hostname property that is either a dns name or an IP address.

This allows users to run https services on AWS gateways without public IPs. Note that users can also use AWS Certificate Manager certificates with public gateways. dstack provisions either internal or internet-facing ALB to support both cases.

Example configuration for provisioning private gateway with https support:

type: gateway
name: test-aws-gateway
backend: aws
region: eu-central-1
domain: 'example.com'
public_ip: False
default: True
certificate:
  type: acm
  arn: 'arn:...'

TODO:

• Allow updating ACM certificate for renewal