feature/logtail: pull logtail + netlog out to modular features

Removes 434 KB from the minimal Linux binary, or ~3%.

Primarily this comes from not linking in the zstd encoding code.

Fixes #17323

Change-Id: I0a90de307dfa1ad7422db7aa8b1b46c782bfaaf7
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

logtail/logtail_omit.go

@@ -0,0 +1,44 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+//go:build ts_omit_logtail
+
+package logtail
+
+import (
+	"context"
+
+	tslogger "tailscale.com/types/logger"
+	"tailscale.com/types/logid"
+)
+
+// Noop implementations of everything when ts_omit_logtail is set.
+
+type Logger struct{}
+
+type Buffer any
+
+func Disable() {}
+
+func NewLogger(cfg Config, logf tslogger.Logf) *Logger {
+	return &Logger{}
+}
+
+func (*Logger) Write(p []byte) (n int, err error) {
+	return len(p), nil
+}
+
+func (*Logger) Logf(format string, args ...any)    {}
+func (*Logger) Shutdown(ctx context.Context) error { return nil }
+func (*Logger) SetVerbosityLevel(level int)        {}
+
+func (l *Logger) SetSockstatsLabel(label any) {}
+
+func (l *Logger) PrivateID() logid.PrivateID { return logid.PrivateID{} }
+func (l *Logger) StartFlush()                {}
+
+func RegisterLogTap(dst chan<- string) (unregister func()) {
+	return func() {}
+}
+
+func (*Logger) SetNetMon(any) {}