listeners: Add network interface binding support, with IP version modes

[Siomachkin]

Implements GitHub issue #6856 - adds support for binding Caddy listeners to network interfaces by name instead of requiring specific IP addresses.

Features

• Interface name binding: Use `bind "eth0" instead of hardcoded IP addresses
• IP version modes: Control IPv4/IPv6 preference with bind "eth0:8080:ipv4", bind "eth0:8080:ipv6", or bind "eth0:8080:auto"

Usage Examples

Basic Interface Binding

example.com {
    bind "eth0"  # Binds to first available IP on eth0 interface
    respond "Hello from eth0!"
}

With Specific Ports and IP Version Modes

# IPv4 only binding
api.example.com {
    bind "eth0:8080:ipv4"
    respond "IPv4 API"
}

# IPv6 only binding
api-v6.example.com {
    bind "eth0:8080:ipv6"
    respond "IPv6 API"
}

# Auto mode (prefers IPv4, falls back to IPv6)
auto.example.com {
    bind "eth0:8080:auto"  # Default mode
    respond "Auto-selected IP"
}

Key Functions

• isInterfaceName(): Validates interface names and handles interface:port:mode patterns
• selectIPByMode(): Chooses appropriate IP address based on binding mode
• resolveInterfaceNameWithMode(): Resolves interface names to IP addresses
• parseInterfaceAddress(): Parses and validates interface binding syntax

Testing

• Unit tests for all parsing and validation logic

Assistance Disclosure
I consulted Claude to understand the project architecture, but I authored/coded the fix myself

[francislavoie]

More interface names to test:

• br-901e40e4488d
• enx9cbf0d00631a
• veth1308dcd
• fe80:: IPv6 that has a starting letter (hex)

[francislavoie]

Don't need all this, NewReplacer() implicitly has that protection for you anyway since it only has those placeholders registered

[Siomachkin]

I added explicit checks for system.* and time.* placeholders since they are still processed by ReplaceKnown() and could produce invalid interface names.