cpp: Expose gpgme_data_set_flag through cpp API

* lang/cpp/src/data.cpp (Data::setFlag): New.
* lang/cpp/src/data.h: Update accordingly.
* NEWS: Mention this.

--
This exposes the generic flag mechanism for data to users
of the C++ library. It is similar to Context::setFlag but
has no getter.

NEWS

@@ -4,6 +4,9 @@ Noteworthy changes in version 1.21.0 (unreleased)
  * Error::isCanceled now also returns true for error code
    GPG_ERR_FULLY_CANCELED.  [T6510]
 
+* Interface changes relative to the 1.20.0 release:
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+ cpp: Data::setFlag                         NEW.
 
 Noteworthy changes in version 1.20.0 (2023-04-20)
 -------------------------------------------------

lang/cpp/src/data.cpp

@@ -280,3 +280,8 @@ std::string GpgME::Data::toString()
   seek (0, SEEK_SET);
   return ret;
 }
+
+GpgME::Error GpgME::Data::setFlag(const char *name, const char *value)
+{
+    return Error(gpgme_data_set_flag(d->data, name, value));
+}

lang/cpp/src/data.h

@@ -122,6 +122,9 @@ public:
     /** Return a copy of the data as std::string. Sets seek pos to 0 */
     std::string toString();
 
+    /** See gpgme_data_set_flag */
+    Error setFlag(const char *name, const char *value);
+
     class Private;
     Private *impl()
     {