Allow wallet2.h to run in WebAssembly

- Add abstract_http_client.h which http_client.h extends.
- Replace simple_http_client with abstract_http_client in wallet2,
message_store, message_transporter, and node_rpc_proxy.
- Import and export wallet data in wallet2.
- Use #if defined __EMSCRIPTEN__ directives to skip incompatible code.

tests/fuzz/http-client.cpp

@@ -29,6 +29,7 @@
 #include "include_base_utils.h"
 #include "file_io_utils.h"
 #include "net/http_client.h"
+#include "net/net_ssl.h"
 #include "fuzzer.h"
 
 class dummy_client
@@ -46,6 +47,10 @@ public:
     data.clear();
     return true;
   }
+  void set_ssl(epee::net_utils::ssl_options_t ssl_options) { }
+  bool is_connected(bool *ssl = NULL) { return true; }
+  uint64_t get_bytes_sent() const  { return 1; }
+  uint64_t get_bytes_received() const { return 1; }
 
   void set_test_data(const std::string &s) { data = s; }