Netsocket: Add get_dns_server API for NetworkInterface

With get_dns_server DNS servers can be queried from NetworkInterface object

UNITTESTS/stubs/NetworkInterface_stub.cpp

@@ -72,6 +72,11 @@ nsapi_error_t NetworkInterface::add_dns_server(const SocketAddress &address, con
     return NSAPI_ERROR_UNSUPPORTED;
 }
 
+nsapi_error_t NetworkInterface::get_dns_server(int index, SocketAddress *address, const char *interface_name)
+{
+    return NSAPI_ERROR_UNSUPPORTED;
+}
+
 void NetworkInterface::attach(mbed::Callback<void(nsapi_event_t, intptr_t)> status_cb)
 {
 

features/netsocket/NetworkInterface.cpp

@@ -101,6 +101,11 @@ nsapi_error_t NetworkInterface::add_dns_server(const SocketAddress &address, con
     return get_stack()->add_dns_server(address, interface_name);
 }
 
+nsapi_error_t NetworkInterface::get_dns_server(int index, SocketAddress *address, const char *interface_name)
+{
+    return get_stack()->get_dns_server(index, address, interface_name);
+}
+
 void NetworkInterface::attach(mbed::Callback<void(nsapi_event_t, intptr_t)> status_cb)
 {
     // Dummy, that needs to be overwritten when inherited, but cannot be removed

features/netsocket/NetworkInterface.h

@@ -324,6 +324,18 @@ public:
      */
     virtual nsapi_error_t add_dns_server(const SocketAddress &address, const char *interface_name);
 
+    /** Get a domain name server from a list of servers to query
+     *
+     *  Returns a DNS server address for a index. If returns error no more
+     *  DNS servers to read.
+     *
+     *  @param index    Index of the DNS server, starts from zero
+     *  @param address  Destination for the host address
+     *  @param interface_name  Network interface name
+     *  @return         NSAPI_ERROR_OK on success, negative error code on failure
+     */
+    virtual nsapi_error_t get_dns_server(int index, SocketAddress *address, const char *interface_name = NULL);
+
     /** Register callback for status reporting.
      *
      *  The specified status callback function will be called on status changes