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.

deltachat-ffi/src/lib.rs

@@ -3425,6 +3425,16 @@ pub unsafe extern "C" fn dc_msg_get_quoted_msg(msg: *const dc_msg_t) -> *mut dc_
     }
 }
 
+#[no_mangle]
+pub unsafe extern "C" fn dc_msg_force_plaintext(msg: *mut dc_msg_t) {
+    if msg.is_null() {
+        eprintln!("ignoring careless call to dc_msg_force_plaintext()");
+        return;
+    }
+    let ffi_msg = &mut *msg;
+    ffi_msg.message.force_plaintext();
+}
+
 // dc_contact_t
 
 /// FFI struct for [dc_contact_t]