Implement part of Berkeley Socket API for libc-wasi (#1036)

Refer to [Networking API design](https://github.com/WebAssembly/WASI/issues/370)
and [feat(socket): berkeley socket API v2](https://github.com/WebAssembly/WASI/pull/459):

- Support the socket API of synchronous mode, including `socket/bind/listen/accept/send/recv/close/shutdown`,
    the asynchronous mode isn't supported yet.
- Support adding `--addr-pool=<pool1,pool2,..>` argument for command line to identify the valid ip address range
- Add socket-api sample and update the document

core/shared/platform/include/platform_api_extension.h

@@ -265,6 +265,17 @@ int
 os_socket_accept(bh_socket_t server_sock, bh_socket_t *sock, void *addr,
                  unsigned int *addrlen);
 
+/**
+ * initiate a connection on a socket
+ *
+ * @param socket the socket to connect with
+ * @param addr the ip address, only IPv4 supported currently
+ *
+ * @return 0 if success, -1 otherwise
+ */
+int
+os_socket_connect(bh_socket_t socket, const char *addr, int port);
+
 /**
  * Blocking receive message from a socket.
  *
@@ -310,6 +321,18 @@ os_socket_close(bh_socket_t socket);
 int
 os_socket_shutdown(bh_socket_t socket);
 
+/**
+ * converts cp into a number in host byte order suitable for use as
+ * an Internet network address
+ *
+ * @param cp a string in IPv4 numbers-and-dots notation
+ *
+ * @return On success, the converted address is  returned.
+ * If the input is invalid, -1 is returned
+ */
+int
+os_socket_inet_network(const char *cp, uint32 *out);
+
 #ifdef __cplusplus
 }
 #endif