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/deltachat.h

@@ -4217,6 +4217,15 @@ char*           dc_msg_get_quoted_text        (const dc_msg_t* msg);
  */
 dc_msg_t*       dc_msg_get_quoted_msg         (const dc_msg_t* msg);
 
+/**
+ * Force the message to be sent in plain text.
+ *
+ * This API is for bots, there is no need to expose it in the UI.
+ *
+ * @memberof dc_msg_t
+ * @param msg The message object.
+ */
+void            dc_msg_force_plaintext        (dc_msg_t* msg);
 
 /**
  * @class dc_contact_t