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/metadata.go

@@ -9,6 +9,11 @@ import (
 
 type metadata struct {
 	handshakeTimeout time.Duration
+
+	keepalive         bool
+	keepaliveIdle     time.Duration
+	keepaliveInterval time.Duration
+	keepaliveCount    int
 }
 
 func (d *tlsDialer) parseMetadata(md mdata.Metadata) (err error) {
@@ -18,5 +23,10 @@ func (d *tlsDialer) parseMetadata(md mdata.Metadata) (err error) {
 
 	d.md.handshakeTimeout = mdutil.GetDuration(md, handshakeTimeout)
 
+	d.md.keepalive = mdutil.GetBool(md, "keepalive")
+	d.md.keepaliveIdle = mdutil.GetDuration(md, "keepalive.idle")
+	d.md.keepaliveInterval = mdutil.GetDuration(md, "keepalive.interval")
+	d.md.keepaliveCount = mdutil.GetInt(md, "keepalive.count")
+
 	return
 }