feat: add possibility do select IP used to connect to server

[Bartosz-lab]

Summary

Adds a new ssh_interface builder option to control which server IP address Packer uses for the communicator connection.

Previously, the builder always selected the first available address in this order:

1. public IPv4
2. public IPv6
3. private IPv4

This could be problematic in environments where public IPv4 is present on the server but not reachable from the machine running Packer. With this change, users can explicitly select the interface used for SSH:

ssh_interface = "public_ipv6"

Supported values are:

• public_ipv4
• public_ipv6
• private_ipv4

When ssh_interface is unset, the existing default behavior is preserved.

Changes

• Added ssh_interface to the hcloud builder config.
• Added validation for supported ssh_interface values.
• Updated server IP selection logic to honor the configured interface.
• Kept the existing first-available-IP behavior as the default.
• Updated HCL2 config spec.
• Documented the new option.
• Added tests for config validation and IP selection.