Support authorization on the gateway

[Egor-S]

Closes #846

• Add auth property to Service configuration. The default value is True
• Require Authorization: Bearer XXX header, where XXX - is a dstack token with access to the project, where the service was created
  • For any service where auth is True
  • For OpenAI endpoint (always)
• Run GatewayConnection on the server for the whole lifetime of a gateway
  • Use unix socket to make requests to the gateway (local tunnel from the server)
  • Use port 8001 to make authorization requests to the server (reverse tunnel from the gateway)
• Replace dicts with jinja2 templates for nginx configurations