Add configurable TCP keepalives to TCP listener and dialer

Exposes TCP keepalive configuration via listener and dialer metadata:

  keepalive: true
  keepalive.idle: 30s
  keepalive.interval: 10s
  keepalive.count: 6

When enabled, the OS sends keepalive probes after the idle period and
tears down the socket if the remote does not respond. This causes any
blocked Read() on a dead connection to return promptly, releasing
goroutines and memory — covering both legs of a CONNECT tunnel (the
inbound client connection via the listener, and the outbound upstream
connection via the dialer).

This is the correct alternative to a hard application-level read
deadline (SetReadDeadline) for detecting dead connections, as it
distinguishes truly dead connections from legitimately idle ones.

Relates to: https://github.com/go-gost/x/issues/91

dialer/tcp/metadata.go

@@ -4,6 +4,7 @@ import (
 	"time"
 
 	md "github.com/go-gost/core/metadata"
+	mdutil "github.com/go-gost/x/metadata/util"
 )
 
 const (
@@ -16,8 +17,17 @@ const (
 
 type metadata struct {
 	dialTimeout time.Duration
+
+	keepalive         bool
+	keepaliveIdle     time.Duration
+	keepaliveInterval time.Duration
+	keepaliveCount    int
 }
 
 func (d *tcpDialer) parseMetadata(md md.Metadata) (err error) {
+	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
 }