cmd/k8s-operator,k8s-operator/sessionrecording,sessionrecording,ssh/tailssh: refactor session recording functionality (#12945)

cmd/k8s-operator,k8s-operator/sessionrecording,sessionrecording,ssh/tailssh: refactor session recording functionality

Refactor SSH session recording functionality (mostly the bits related to
Kubernetes API server proxy 'kubectl exec' session recording):

- move the session recording bits used by both Tailscale SSH
and the Kubernetes API server proxy into a shared sessionrecording package,
to avoid having the operator to import ssh/tailssh

- move the Kubernetes API server proxy session recording functionality
into a k8s-operator/sessionrecording package, add some abstractions
in preparation for adding support for a second streaming protocol (WebSockets)

Updates tailscale/corp#19821

Signed-off-by: Irbe Krumina <irbe@tailscale.com>

k8s-operator/sessionrecording/conn/conn.go

@@ -0,0 +1,20 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+//go:build !plan9
+
+// Package conn contains shared interface for the hijacked
+// connection of a 'kubectl exec' session that is being recorded.
+package conn
+
+import "net"
+
+type Conn interface {
+	net.Conn
+	// Fail can be called to set connection state to failed. By default any
+	// bytes left over in write buffer are forwarded to the intended
+	// destination when the connection is  being closed except for when the
+	// connection state is failed- so set the state to failed when erroring
+	// out and failure policy is to fail closed.
+	Fail()
+}