Message: add set_opt_field and remove_opt_field, making opt fields modifiable from Python

Also update DEPENDENCIES to use a new Engine RC with the new (trivial) API
functions.

DEPENDENCIES

@@ -1,6 +1,6 @@
 # 1st Party Dependencies
 ## Prefer git tags instead of SHA hashes when possible.
 
-pEpEngine=v3.2.0-RC27
+pEpEngine=v3.3.0-RC7
 libpEpAdapter= v3.2.0-RC5
 libpEpCxx=v3.2.0-RC1

src/pEp/_pEp/message.cc

@@ -300,6 +300,20 @@ namespace pEp {
             _msg->attachments = bl;
         }
 
+        void Message::remove_opt_field(string name)
+        {
+            ::message *msg_c = _msg.get();
+            PEP_STATUS status = ::message_remove_opt_field(Adapter::session(), msg_c, name.c_str());
+            _throw_status(status);
+        }
+
+        void Message::set_opt_field(string name, string value)
+        {
+            ::message *msg_c = _msg.get();
+            PEP_STATUS status = ::message_set_opt_field(Adapter::session(), msg_c, name.c_str(), value.c_str());
+            _throw_status(status);
+        }
+
         Message Message::encrypt()
         {
             boost::python::list extra;

src/pEp/_pEp/message.hh

@@ -300,6 +300,9 @@ namespace pEp {
                 _msg->enc_format = value;
             }
 
+            void remove_opt_field(string name);
+            void set_opt_field(string name, string value);
+
             Message encrypt();
 
             Message _encrypt(boost::python::list extra, int enc_format = 4, int flags = 0);

src/pEp/_pEp/pEpmodule.cc

@@ -535,6 +535,22 @@ namespace pEp {
                                          (void(Message::*)(int))(void(Message::*)(PEP_enc_format)) &
                                              Message::enc_format,
                                          "0: unencrypted, 1: inline PGP, 2: S/MIME, 3: PGP/MIME, 4: p≡p format")
+                                     .def("remove_opt_field", &Message::remove_opt_field,
+                                          "msg.remove_opt_field(name)\n"
+                                          "\n"
+                                          "alters msg removing an opt_field with the given name, if present.\n"
+                                          "\n"
+                                          "   msg            a message\n"
+                                          "   name           the field name as a string\n")
+                                     .def("set_opt_field", &Message::set_opt_field,
+                                          "msg.set_opt_field(name, value)\n"
+                                          "\n"
+                                          "alters msg replacing an opt_field, or adding it, if the same field\n"
+                                          "was not present before.\n"
+                                         "\n"
+                                          "   msg            a message\n"
+                                          "   name           the field name as a string\n"
+                                          "   value          the new field value as a string\n")
                                      .def("encrypt", (Message(Message::*)()) & Message::encrypt)
                                      .def(
                                          "encrypt",