Add destination address mappings

[SoundGoof]

Summary

Add hostname-to-IP destination mappings so clients request an RDP server by FQDN while rdpgw connects to a configured IP address.

Configuration

Server:
  HostSelection: unsigned
  Destinations:
    - Hostname: rdp.example.com
      Address: 192.0.2.42
      Port: 3389

Port defaults to 3389. IPv4 and IPv6 are supported.

Behavior

• Authorizes connections using the requested FQDN.
• Dials the configured IP address.
• Preserves the FQDN for RDP certificate validation.
• Adds mapped destinations to the effective host list.
• Remains compatible with legacy Server.Hosts.
• Normalizes hostname case and trailing DNS dots.
• Rejects duplicate or malformed mappings during startup.
• Treats explicit mappings as curated exceptions in HostSelection: any.

Security

• Enforces dynamic destination policy at channel dial time.
• Dials validated IPs to prevent DNS rebinding.
• Applies context-aware DNS and TCP timeouts.
• Uses staggered fallback across resolved addresses.
• Preserves resolver cancellation and timeout errors.

Testing

• make build
• make test
• go vet ./cmd/rdpgw/...
• Focused race tests for resolver and dial fallback paths.
• git diff --check