Add PPP driver

src/drivers/ppp.h

@@ -0,0 +1,11 @@
+#pragma once
+
+struct mg_tcpip_driver_ppp_data {
+  void *uart;                   // Opaque UART bus descriptor
+  void (*reset)(void *);        // Modem hardware reset
+  void (*tx)(void *, uint8_t);  // UART transmit single byte
+  int (*rx)(void *);            // UART receive single byte
+  const char **script;          // List of AT commands and expected replies
+  int script_index;             // Index of the current AT command in the list
+  uint64_t deadline;            // AT command deadline in ms
+};