feat: extend TCP keepalive support to all TCP-based listeners and dialers

Add configurable TCP keepalive (keepalive, keepalive.idle,
keepalive.interval, keepalive.count) to all TCP-based transport types:
tls, mtls, mtcp, ws/wss, mws/mwss, http2, ssh, sshd listeners and
tls, mtls, mtcp, ws/wss, mws/mwss, ssh, sshd dialers.

For types that already use "keepalive" for an app-level protocol
(grpc listener/dialer, ws/mws dialers, ssh/sshd dialers), the TCP
keepalive parameters are exposed under the tcp.keepalive.* prefix to
avoid ambiguity.

Also extract shared helpers into internal/net/keepalive.go:
- WrapKeepaliveListener: applies KeepAliveConfig on every accepted TCPConn
- ApplyKeepalive: applies KeepAliveConfig to a single TCPConn

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

dialer/tls/dialer.go

@@ -10,6 +10,7 @@ import (
 	"github.com/go-gost/core/logger"
 	md "github.com/go-gost/core/metadata"
 	xctx "github.com/go-gost/x/ctx"
+	xnet "github.com/go-gost/x/internal/net"
 	"github.com/go-gost/x/internal/net/proxyproto"
 	"github.com/go-gost/x/registry"
 )
@@ -51,6 +52,15 @@ func (d *tlsDialer) Dial(ctx context.Context, addr string, opts ...dialer.DialOp
 		d.log.Error(err)
 	}
 
+	if d.md.keepalive {
+		xnet.ApplyKeepalive(conn, net.KeepAliveConfig{
+			Enable:   true,
+			Idle:     d.md.keepaliveIdle,
+			Interval: d.md.keepaliveInterval,
+			Count:    d.md.keepaliveCount,
+		})
+	}
+
 	conn = proxyproto.WrapClientConn(
 		d.options.ProxyProtocol,
 		xctx.SrcAddrFromContext(ctx),