Add dc_msg_force_plaintext() API for bots (#2847)

This will allow implementing a special command for download bot to request that it sends back the downloaded file unencrypted.

src/e2ee.rs

@@ -502,7 +502,7 @@ Sent with my Delta Chat Messenger: https://delta.chat";
 
         // Alice sends plaintext message with Autocrypt header.
         let mut msg = Message::new(Viewtype::Text);
-        msg.param.set_int(Param::ForcePlaintext, 1);
+        msg.force_plaintext();
         chat::prepare_msg(&alice.ctx, chat_alice, &mut msg).await?;
         chat::send_msg(&alice.ctx, chat_alice, &mut msg).await?;
         let sent = alice.pop_sent_msg().await;
@@ -516,7 +516,7 @@ Sent with my Delta Chat Messenger: https://delta.chat";
 
         // Alice sends plaintext message without Autocrypt header.
         let mut msg = Message::new(Viewtype::Text);
-        msg.param.set_int(Param::ForcePlaintext, 1);
+        msg.force_plaintext();
         msg.param.set_int(Param::SkipAutocrypt, 1);
         chat::prepare_msg(&alice.ctx, chat_alice, &mut msg).await?;
         chat::send_msg(&alice.ctx, chat_alice, &mut msg).await?;

src/imex.rs

@@ -238,7 +238,7 @@ async fn do_initiate_key_transfer(context: &Context) -> Result<String> {
     msg.param
         .set(Param::MimeType, "application/autocrypt-setup");
     msg.param.set_cmd(SystemMessage::AutocryptSetupMessage);
-    msg.param.set_int(Param::ForcePlaintext, 1);
+    msg.force_plaintext();
     msg.param.set_int(Param::SkipAutocrypt, 1);
 
     let msg_id = chat::send_msg(context, chat_id, &mut msg).await?;

src/message.rs

@@ -893,6 +893,11 @@ impl Message {
         Ok(None)
     }
 
+    /// Force the message to be sent in plain text.
+    pub fn force_plaintext(&mut self) {
+        self.param.set_int(Param::ForcePlaintext, 1);
+    }
+
     pub async fn update_param(&self, context: &Context) {
         context
             .sql

src/securejoin/bobstate.rs

@@ -422,7 +422,7 @@ impl BobState {
             BobHandshakeMsg::Request => {
                 // Sends the Secure-Join-Invitenumber header in mimefactory.rs.
                 msg.param.set(Param::Arg2, self.invite.invitenumber());
-                msg.param.set_int(Param::ForcePlaintext, 1);
+                msg.force_plaintext();
             }
             BobHandshakeMsg::RequestWithAuth => {
                 // Sends the Secure-Join-Auth header in mimefactory.rs.