Add option to define supported IP stacks

[DerAndereAndi]

Right now the implementation will always announce IPv4 and IPv6 on the defined or all network interfaces. It would be great if the user could also define if All, only IPv4 or only IPv6 should be used.

[DerAndereAndi]

With v3 this could look something like this:

Issue: The Register function automatically discovers and announces both IPv4 and IPv6 addresses. For protocols that are IPv6-only (like Matter and MASH), announcing IPv4 addresses is unnecessary and potentially misleading.

Location: server.go lines 105-109

Current code:

for _, iface := range ifaces {
    v4, v6 := addrsForInterface(&iface)
    entry.AddrIPv4 = append(entry.AddrIPv4, v4...)
    entry.AddrIPv6 = append(entry.AddrIPv6, v6...)
}

Problem: No way to register a service with only IPv6 addresses without using RegisterProxy (which requires manual IP discovery and hostname management).

Proposal: Add a ServerOption to filter by IP family:

type IPFamily int

const (
    IPFamilyAll  IPFamily = iota // Default: both IPv4 and IPv6
    IPFamilyIPv4                 // IPv4 only
    IPFamilyIPv6                 // IPv6 only
)

func WithIPFamily(family IPFamily) ServerOption {
    return func(o *serverOpts) {
        o.ipFamily = family
    }
}

Then in Register:

for _, iface := range ifaces {
    v4, v6 := addrsForInterface(&iface)
    if conf.ipFamily != IPFamilyIPv6 {
        entry.AddrIPv4 = append(entry.AddrIPv4, v4...)
    }
    if conf.ipFamily != IPFamilyIPv4 {
        entry.AddrIPv6 = append(entry.AddrIPv6, v6...)
    }
}

Workaround: Use RegisterProxy with manually discovered IPv6 addresses.