Allow updating gateway ACM certificate

[r4victor]

#1264 added support for gateways with ACM certificates. Since ACM certificates are not renewed by dstack automatically, users should be able to update them in the gateway configuration. This should be as easy as specifying a new ARN in the configuration and running dstack apply. dstack apply should detect that the certificate has changed and update it.

Implementation details:

AWSCompute will likely get a special method for updating the ACM certificate on the gateway. It can be done by updating the listener:

response = elb_client.modify_listener(
        ListenerArn=listener_arn,
        Certificates=[
            {
                'CertificateArn': new_certificate_arn
            }
        ]
    )

[peterschmidt85]

This issue is stale because it has been open for 30 days with no activity.