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/grpc/metadata.go

@@ -16,6 +16,11 @@ type metadata struct {
 	keepaliveTimeout             time.Duration
 	keepalivePermitWithoutStream bool
 	minConnectTimeout            time.Duration
+
+	tcpKeepalive         bool
+	tcpKeepaliveIdle     time.Duration
+	tcpKeepaliveInterval time.Duration
+	tcpKeepaliveCount    int
 }
 
 func (d *grpcDialer) parseMetadata(md mdata.Metadata) (err error) {
@@ -39,5 +44,10 @@ func (d *grpcDialer) parseMetadata(md mdata.Metadata) (err error) {
 		d.md.minConnectTimeout = 30 * time.Second
 	}
 
+	d.md.tcpKeepalive = mdutil.GetBool(md, "tcp.keepalive")
+	d.md.tcpKeepaliveIdle = mdutil.GetDuration(md, "tcp.keepalive.idle")
+	d.md.tcpKeepaliveInterval = mdutil.GetDuration(md, "tcp.keepalive.interval")
+	d.md.tcpKeepaliveCount = mdutil.GetInt(md, "tcp.keepalive.count")
+
 	return
 }