[Bug]: Failed to create private AWS gateway if there are multiple private subnets per AZ

[r4victor]

Steps to reproduce

1. Set up an AWS VPC with multiple private subnets connected to NAT gateway
2. Try to create a private dstack gateway on AWS:

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

You'll get

Provisioning failed. Error: Unexpected error: InvalidConfigurationRequestException('An error occurred
(InvalidConfigurationRequest) when calling the CreateLoadBalancer operation: A load balancer cannot be attached to multiple
subnets in the same Availability Zone')

Actual behaviour

No response

Expected behaviour

Filter out subnets if there are multiple eligible subnets per AZ.

dstack version

master

Server logs

Additional information

No response