Improve gateway security, show verbose errors

[Egor-S]

• All gateway logic implemented in Hub
• Runner uses www-data gateway user without permission to execute any commands
• Hub detects conflicts between services requesting the same hostname and port, giving an error to the second service
• Service could be published on any port (80 by default)
• Simplify service configuration
• gateway field supports secrets interpolation on the Hub side
• SSH errors passed to the CLI with meaningful messages
• Docs updated

Breaking changes, you have to recreate your gateways

Service configuration example:

dstack secrets add GATEWAY my.gateway.domain.com

type: service
gateway: ${{ secrets.GATEWAY }}
port: 8000  # the same as 80:8000
commands:
  - python -m http.server 8000

Service would be available at my.gateway.domain.com:80