Support IPv6 subnets in --net-restrict

[usmansaleem]

Description

--net-restrict currently only supports IPv4 CIDR subnets because it relies on Apache Commons Net SubnetUtils/SubnetInfo, which is IPv4-only. When a peer has an IPv6 address and --net-restrict is configured, SubnetInfo.isInRange() throws IllegalArgumentException on the IPv6 address string.

This affects both DiscV4 (PeerPermissionSubnet) and DiscV5 (PeerDiscoveryAgentFactoryV5.createAddressAccessPolicy()).

Impact

Scenario	Behavior
IPv4 only	✅ Works correctly
Dual-stack, peer on IPv4	✅ Works correctly
Dual-stack, peer on IPv6	❌ isInRange() throws on IPv6 address
IPv6 only	❌ No subnet filtering possible

Proposed Solution

Replace SubnetUtils.SubnetInfo with a dual-stack CidrBlock abstraction that handles both IPv4 and IPv6 CIDR notation:

1. Create CidrBlock class using java.net.InetAddress + byte-level prefix comparison (~50 lines)
2. Update SubnetInfoConverter to parse both IPv4 (192.168.0.0/24) and IPv6 (fd00::/64) CIDR
3. Replace SubnetInfo with CidrBlock across all usages (~11 files)
4. Handle edge cases: IPv4-mapped IPv6 addresses (::ffff:192.168.1.1), mixed v4/v6 CIDR lists

Affected files

• app/.../cli/converter/SubnetInfoConverter.java
• app/.../cli/options/P2PDiscoveryOptions.java
• ethereum/p2p/.../permissions/PeerPermissionSubnet.java
• ethereum/p2p/.../discovery/discv5/PeerDiscoveryAgentFactoryV5.java
• ethereum/p2p/.../discovery/DefaultPeerDiscoveryAgentFactory.java
• ethereum/p2p/.../discovery/P2PDiscoveryConfiguration.java
• app/.../RunnerBuilder.java
• Related test files