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/buffer.go

@@ -1,6 +1,8 @@
 // Copyright (c) Tailscale Inc & AUTHORS
 // SPDX-License-Identifier: BSD-3-Clause
 
+//go:build !ts_omit_logtail
+
 package logtail
 
 import (