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>

cmd/tsconnect/wasm/wasm_js.go

@@ -110,6 +110,7 @@ func newIPN(jsConfig js.Value) map[string]any {
 		SetSubsystem:  sys.Set,
 		ControlKnobs:  sys.ControlKnobs(),
 		HealthTracker: sys.HealthTracker.Get(),
+		ExtraRootCAs:  sys.ExtraRootCAs,
 		Metrics:       sys.UserMetricsRegistry(),
 		EventBus:      sys.Bus.Get(),
 	})