feat(healthcheck): HEALTH_TARGET_ADDRESS -> HEALTH_TARGET_ADDRESSES

- Specify fallback addresses
- Defaults changed from cloudflare:443 to cloudflare:443,github.com:443
- Startup check runs a parallel dial to each of the addresses specified with a global 6s timeout
- Full periodic check cycles through addresses as it fails and moves to retry

internal/vpn/interfaces.go

@@ -101,7 +101,7 @@ type CmdStarter interface {
 }
 
 type HealthChecker interface {
-	SetConfig(tlsDialAddr string, icmpTarget netip.Addr)
+	SetConfig(tlsDialAddrs []string, icmpTarget netip.Addr)
 	Start(ctx context.Context) (runError <-chan error, err error)
 	Stop() error
 }