[dns] signal an error immediately when no upstream DNS server available (#11480)

Currently, if the platform has no upstream DNS server configured,
upstream DNS query will eventually time out. This can lead to delays
in responding to the DNS client.

The new otPlatDnsIsUpstreamQueryAvailable API provides a mechanism for
the platform to proactively signal the unavailability of upstream DNS.

tests/unit/test_platform.cpp

@@ -700,6 +700,13 @@ otError otPlatUdpLeaveMulticastGroup(otUdpSocket        *aUdpSocket,
 #endif // OPENTHREAD_CONFIG_PLATFORM_UDP_ENABLE
 
 #if OPENTHREAD_CONFIG_DNS_UPSTREAM_QUERY_ENABLE
+bool otPlatDnsIsUpstreamQueryAvailable(otInstance *aInstance)
+{
+    OT_UNUSED_VARIABLE(aInstance);
+
+    return true;
+}
+
 void otPlatDnsStartUpstreamQuery(otInstance *aInstance, otPlatDnsUpstreamQuery *aTxn, const otMessage *aQuery)
 {
     OT_UNUSED_VARIABLE(aInstance);