tsd, all: add Sys.ExtraRootCAs, plumb through TLS dial paths

Add ExtraRootCAs *x509.CertPool to tsd.System and plumb it through
the control client, noise transport, DERP, and wgengine layers so
that platforms like Android can inject user-installed CA certificates
into Go's TLS verification.

tlsdial.Config now honors base.RootCAs as additional trusted roots,
tried after system roots and before the baked-in LetsEncrypt fallback.
SetConfigExpectedCert gets the same treatment for domain-fronted DERP.

The Android client will set sys.ExtraRootCAs with a pool built from
x509.SystemCertPool + user-installed certs obtained via the Android
KeyStore API, replacing the current SSL_CERT_DIR environment variable
approach.

Updates #8085

Change-Id: Iecce0fd140cd5aa0331b124e55a7045e24d8e0c2
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

control/controlhttp/constants.go

@@ -4,6 +4,7 @@
 package controlhttp
 
 import (
+	"crypto/x509"
 	"net/http"
 	"net/url"
 	"sync/atomic"
@@ -85,6 +86,9 @@ type Dialer struct {
 	// HealthTracker, if non-nil, is the health tracker to use.
 	HealthTracker *health.Tracker
 
+	// ExtraRootCAs, if non-nil, specifies additional trusted root CAs for TLS.
+	ExtraRootCAs *x509.CertPool
+
 	// DialPlan, if set, contains instructions from the control server on
 	// how to connect to it. If present, we will try the methods in this
 	// plan before falling back to DNS.