feature/acme, ipn/ipnlocal: start moving ACME/cert state into an extension

The ACME serialization mutex (acmeMu) was a package-level global, and
several ACME-related fields lived on LocalBackend even though the
cert code is conditional and not linked into every binary. With
multiple tsnet.Servers in one process (each its own LocalBackend),
a process-wide acmeMu also serialized unrelated backends.

Introduce a new feature/acme extension that owns the per-LocalBackend
ACME/cert state in an ipnlocal.CertState value:

  - acmeMu, renewMu, renewCertAt (previously package globals)
  - pendingACMETLSALPNCerts, pendingCertDomains{,Mu},
    getCertForTest, certRefreshCancel (previously LocalBackend
    fields, only meaningful when ACME was compiled in)

ipnlocal/cert.go now reaches the state through b.certState(), which
is routed by a feature.Hook installed at init by feature/acme. The
CertState type lives in ipnlocal so cert.go can access its fields
directly without a method explosion; the extension in feature/acme
constructs and owns it.

This is a baby step. The end goal is for the entire cert/ACME code
to live in feature/acme, with ipnlocal only retaining whatever thin
hooks the rest of LocalBackend needs to call into it. The current
split (CertState and most of cert.go in ipnlocal, extension wrapper
in feature/acme) is a deliberately temporary middle ground that
keeps this PR small while making the next moves mechanical.

The package is named feature/acme to match the existing HasACME /
ts_omit_acme naming. condregister/maybe_acme.go wires it in for
non-js builds.

Updates #12614
Updates #20248
Updates #20249

Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
Change-Id: I520909f24ad11a9622ef33c2290fe36ad44d6f71

feature/condregister/maybe_acme.go

@@ -0,0 +1,8 @@
+// Copyright (c) Tailscale Inc & contributors
+// SPDX-License-Identifier: BSD-3-Clause
+
+//go:build !js && !ts_omit_acme
+
+package condregister
+
+import _ "tailscale.com/feature/acme"