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

ipn/ipnlocal/cert_test.go

@@ -586,12 +586,6 @@ func TestCertStoreRoundTrip(t *testing.T) {
 }
 
 func TestShouldStartDomainRenewal(t *testing.T) {
-	reset := func() {
-		renewMu.Lock()
-		defer renewMu.Unlock()
-		clear(renewCertAt)
-	}
-
 	mustMakePair := func(template *x509.Certificate) *TLSCertKeyPair {
 		priv, err := ecdsa.GenerateKey(elliptic.P256(), rand.Reader)
 		if err != nil {
@@ -653,8 +647,6 @@ func TestShouldStartDomainRenewal(t *testing.T) {
 	b := new(LocalBackend)
 	for _, tt := range testCases {
 		t.Run(tt.name, func(t *testing.T) {
-			reset()
-
 			ret, err := b.domainRenewalTimeByExpiry(mustMakePair(&x509.Certificate{
 				SerialNumber: big.NewInt(2019),
 				Subject:      subject,