feat(quic): add experimental client-side connection migration support

[willianpaixao]

This is for testing purposes, do not merge

#3446 explains what I'm trying to archive with this PR.

Usage

Enable migration support

import libp2pquic "github.com/libp2p/go-libp2p/p2p/transport/quic"

host, err := libp2p.New(
    libp2p.Transport(libp2pquic.NewTransport,
        libp2pquic.EnableExperimentalConnectionMigration()),
)

Perform a migration

// Get MigratableConn from any connection
var migratable network.MigratableConn
if conn.As(&migratable) && migratable.SupportsMigration() {
    // Add a new path using a different local address
    path, err := migratable.AddPath(ctx, newLocalAddr)
    if err != nil {
        return err
    }

    // Probe the path to verify connectivity
    if err := path.Probe(ctx); err != nil {
        path.Close()
        return err
    }

    // Switch to the new path
    if err := path.Switch(); err != nil {
        return err
    }
    // Connection is now using the new path, all streams preserved
}

Monitor migration events

sub, _ := host.EventBus().Subscribe([]any{
    new(event.EvtConnectionMigrationStarted),
    new(event.EvtConnectionMigrationCompleted),
    new(event.EvtConnectionMigrationFailed),
})

Constraints

Per the QUIC specification:

• Only client-initiated (outbound) connections support migration: server-side connections return SupportsMigration() == false
• Single active path at a time: QUIC maintains one active path per connection
• Requires separate UDP socket per interface: each local address needs its own transport

Failure Handling

If migration fails:

1. The connection attempts to continue on the original path
2. If the original path is also unavailable, the connection is closed
3. EvtConnectionMigrationFailed event is emitted with error details

Future Work (Out of scope)

• Automatic migration based on OS network interface events
• Automatic retry with exponential backoff on transient failures
• Policy configuration for migration preferences

[MarcoPolo]

Thanks for the PR. Let's use the existing Conn.As functionality for this use case for now.